Proc. Nail. Acad. Sci. USA Vol. 87, pp. 2856-2860, April 1990 Immunology Genomic organization of mouse Fcy receptor genes (Fc receptors/exon-ntron junctions/protein domains/immunoglobulin gene superfamily) ANTHONY KULCZYCKI, JR.*t, JENNIFER WEBBER*, HAUNANI A. SOARES*, MICHAEL D. ONKEN*, JAMES A. THOMPSON*, DAVID D. CHAPLIN*t, DENNIS Y. LOH*t, AND JEFFREY P. TILLINGHAST* *Department of Medicine and tHoward Hughes Medical Institute, Washington University School of Medicine, Saint Louis, MO 63110 Communicated by David M. Kipnis, January 16, 1990

ABSTRACT We have isolated and characterized the gene responsible for intrathymic deletion of a large fraction of coding for the mouse Fc receptor that is termed Fc,,RHa. The potentially self-reactive T-cell clones (11-13). gene contains five exons and spans approximately 9 kilobases. An understanding of the relationship of the various Fc Unlike most members of the immunoglobulin gene superfam- receptor genes to each other, the factors influencing their ily, this gene utilizes multiple exons to encode its leader peptide. evolution, and molecular mechanisms regulating their The first exon encodes the hydrophobic region of the signal expression requires a detailed analysis of the structures of sequence; the second exon, which contains only 21 base pairs, their genes. Here we report the characterization of two encodes a segment of the signal peptidase recognition site; and members of this gene family. the beginning of the third exon encodes the predicted site of peptidase cleavage. The third and fourth exons each code for immunoglobulin-like extracellular domains. The fifth exon MATERIALS AND METHODS encodes the hydrophobic transmembrane domain and the Screening of Mouse cDNA and Genomic Libraries. A NZW cytoplasmic tail. Partial characterization of the FcyRIIb gene mouse thymus cDNA library in phage AgtlO was screened by indicates that it also contains multiple leader exons, including using the 32P-end-labeled oligonucleotide probes 5'-GGA- a 21-base-pair exon and two exons coding for homologous CCTGGCTCCGGATGGACCTCCCATTGTGGAAC- immunoglobulin-like extracellular domains. However, the CACTG-3' and 5'-GAAATAAAGGCCCGTGTCCACTG- Fc,,Rlb gene uses four exons to encode its intracytoplasmic CAAACAGGAGGCACATC-3', which correspond to region. Analysis using contour-clamped homogeneous electric FcrRIIb1 cDNA and FcRIIa cDNA (nucleotides 544-583 field (CHEF) gels indicates that the FcRlIa and FcRIlb genes and 723-762, respectively) (3). Two NZW cDNA clones were are linked within 160 kilobases on mouse chromosome 1. isolated. One clone, "a," contained an insert of 0.8 kilobase (kb) specific for FcyRIIa starting at "FcyRa" nucleotide 706 Fc receptors are a family of cell surface proteins that bind to (3). A second clone, "b," contained a 1.5-kb insert that the Fc regions of immunoglobulins and thereby enable anti- corresponds to "FcrRI31," starting at nucleotide 273 and gen-antibody interactions to influence cellular responses (for including 510 nucleotides with 96.7% identity between reviews, see refs. 1 and 2). Interestingly, not only can FcrRIIb and FcrRIIa (3). distinctive Fc receptor genes be expressed on different cell The 1.5-kb cDNA insert of clone b was labeled with 32P by types, but also different spliced messages from the same Fc the random priming method (14) and was used to screen two receptor gene can be expressed in a tissue-specific manner genomic libraries: a mouse BALB/c library in EMBL-3 (1-6). The result of antigen binding to antibody molecules (Clontech), and a mouse BALB/c library in the cosmid that are bound to different Fc receptors is also dependent vector pTCF (15). Screening, hybridization, and washing upon cell type (1, 2)-e.g., ingestion and processing of procedures were carried out as described (16-18). Isolated antigen in macrophages (6), regulation of antibody synthesis colonies were rescreened with an Fc,,RIIa-specific probe (a in B cells, and triggering ofhistamine release from mast cells. 0.42-kb Bgl I fragment of the cDNA insert of clone a). Mouse macrophages express three distinct types (1) of Fc Initial screening of 500,000 plaques of the BALB/c ge- receptors specific for IgG (FcR): one (FczRI) that specifi- nomic library in EMBL-3 yielded three independent clones cally binds monomeric IgG2a; a second (FcrRII) that binds containing exons 4 and 5 of the FcrRIIa gene (see Fig. 1) and aggregated IgG1, IgG2a, and IgG2b (7, 8); and a third three independent clones containing portions of the FcYRIIb FcRIII) that binds only the minor subclass, IgG3. Two gene (see Fig. 3). From a BALB/c genomic cosmid library of different but homologous cDNAs (FcyRIIa and FcyRIIb) 300,000 colonies, four independent overlapping cosmid each encode receptors for aggregated IgG (2-5). The gene clones were isolated, and three of these clones contained all encoding FcYRIIa is expressed only in macrophages (3, 4), five exons of the FcrRIIa gene. and its expression is selectively induced by y interferon (6). Characterization ofGenomic Clones. DNAs from phage and The gene encoding FcRIIb is expressed in both macro- cosmid clones and specific subcloned fragments were char- phages and lymphocytes, with alternative splicing producing acterized by standard restriction endonuclease mapping and two transcripts, b, and b2, which differ only in' their cyto- Southern blot analyses. DNA sequencing of specific restric- plasmic regions (3-5). The genes for FcYR that have been tion fragments subcloned into pBluescript SK vectors (Strat- cloned (FcRIIa and Fc7RIIb genes) are genetically insepa- agene) was performed by using the chain-termination se- rable from the Mls-i locus on mouse chromosome 1 (9-11). quencing method. Coding regions and exon-intron junctions The Mls-i locus controls B-cell products that stimulate mixed were sequenced in both directions by using T3 and T7 lymphocyte reactions in H-2-compatible strains and, with the promoter primers of pBluescript SK and synthetic oligonu- class II major histocompatibility complex molecule I-E, are cleotides (20-mers) corresponding to known exon or intron

The publication costs of this article were defrayed in part by page charge Abbreviations: FcR, Fc receptor(s) specific for IgG; CHEF, con- payment. This article must therefore be hereby marked "advertisement" tour-clamped homogeneous electric field. in accordance with 18 U.S.C. §1734 solely to indicate this fact. tTo whom reprint requests should be addressed. 2856 Downloaded by guest on September 30, 2021 Immunology: Kulczycki et al. Proc. Natl. Acad. Sci. USA 87 (1990) 2857

1kb Fc1R]b A|T C T T G C T G C T GGGA C T C A Fc1Rla c TET T GC GACAGGCAGAGTG

FceR oc chain C T G T GT CA TAACAW RB R H B B H H R )I - D Leu Ala Ala Gly Thr His (Asp) exons 1 2 3 4 5 FcRllb (Asn) FcRla (Ala) Phe Ala Asp Arg Gln Ser (Ala) [AIa ; Leu] FcER os chain (Ser) Leu Gly Val Met Leu Thr (Ala) B 3.7 . B 3.9 P. R 2.5 I- R 10.1 . l FIG. 2. Comparison of the 21-bp exons of the FcYRIIb, FcYRIIa, and FcRI a-chain genes and the segments of signal sequence that FIG. 1. Structure of the mouse gene. Exons are num- FcYRIIa they encode. Nucleotide homologies are indicated in boxes. Amino bered and shown as boxes. All restriction sites forBamHl (B), EcoRI acids in parentheses are encoded the exons, (R), and HindIll (H) are indicated. The four subclones used for partially by 21-bp whereas bracketed amino acids are encoded by the adjacent exon. are labeled their restriction site bound- mapping and sequencing by Arrows indicate preferred predicted sites of peptidase cleavage for aries and size. Exon 1 is represented from the start of translation. Fc,,RIIb (3-5), FczRIIa (see text), and FcRI a-chain (25) gene products. sequences. Nucleotide sequences were analyzed by using the MicroGenie program (Beckman). lular segment, the transmembrane and cytoplasmic regions, Contour-Clamped Homogeneous Electric Field (CHEF) and the 3' noncoding region. The exons range in size from 21 Gels. Mouse J774 DNA was prepared in low-gelling- bp (exon 2) to 648 bp (exon 5). All nucleotides in the coding temperature agarose as described (19), and aliquots were region for mouse Fc,,RIIa match the published cDNA clone digested with restriction enzymes. Samples were electropho- (3). resed in 1% agarose gels with a CHEF hexagonal array Introns range in size from 0.66 kb (intron A) and 0.85 kb apparatus (20) for 28 hr. Field direction was reoriented by (intron B) to 3.5 kb (intron C). The shortest introns flank the using a switch ramp from 0.3 to 15 sec with the field strength smallest exon. The poly(A) signal AATAAA (27) is found in at 6 V/cm. Gels were hybridized at 65°C with 32P-labeled exon 5 (nucleotides 1307-1312). Fc,,RIIb-specific (Apa I/Bgl I) or FcYRIIa-specific cDNA Partial Map ofthe Gene Encoding FcRIIb. In screening the probes and were washed with 0.5x SSC at 650C (lx = 0.15 BALB/c genomic library in EMBL-3, we isolated three M NaCI/0.015 M sodium citrate, pH 7). clones that contained portions ofthe FcYRIIb gene. A partial map of the FcYRIIb gene was constructed from these clones RESULTS (Fig. 3) and includes six of the exons. The FcYRIIb gene, like the Fc,,RIIa gene, contains a 21-bp exon that encodes a Organization and Sequence of the Mouse Gene Encoding portion of the leader peptide required for the peptidase FclRla. A restriction map derived from cosmid clones of cleavage site [Fig. 2, top (Fc,,RIIb) sequences; also denoted genomic DNA containing the FczRIIa gene is shown in Fig. L* in Fig. 3]. The FcYRIIb gene also contains two exons that 1. The FcRIIa gene consists of five exons distributed over encode immunoglobulin-like extracellular domains (EC1 and -9 kb. The exon-intron boundaries and all coding regions of EC2). Unlike the FcYRIIa gene, the FcYRIIb gene contains the FcYRIIa gene were sequenced (Table 1). All introns three exons coding only for cytoplasmic segments of the conform to the GT-AG rule (21), and surrounding sequences receptor (Cl, C2, and C3). The alternative splicing event are closely related to the consensus sequences surrounding previously described for FcYRIIb (3) involves elimination of splicejunctions (22). Exon 1 encodes the 5' noncoding region the 141-nucleotide C1 segment. The exon-intron boundaries and most of the leader peptide. Exon 2 is an unusually small thus far determined are shown in Table 2. Based on cDNA 21-base-pair (bp) exon encoding amino acids that form part of sequence (3-5), at least two additional exons must be pres- the signal peptidase recognition site (23, 24). The 5' end of ent, one or more to encode the remainder of the leader exon 3 also participates in coding for the amino acids bor- peptide and at least one to encode the transmembrane portion dering the signal peptidase cleavage site [Fig. 2, middle with short adjacent intracytoplasmic and extracellular seg- (FcRIIa) sequences]. Exons 3 and 4 each encode a single ments (TM). extracellular domain with the characteristics of the immuno- Restriction fragments from genomic DNA that were de- globulin superfamily (26). Exon 5 encodes a small extracel- tected by various 32P-labeled FczRII cDNA probes in South- Table 1. Exon-intron junctions of the mouse FcRIIa gene Exon Intron No. Size, bp 5' splice donor Designation Size, kb 3' splice acceptor 130 1 130* CTGTTTG gtgagt A 0.66 tcttotttacag CTTTTGCA LeuPheA laPheAla 151 2 21 CAGAGTG gtaagt B 0.85 tctcactotcag CAGCT CTT GinSerA laAla Leu 409 3 258 ATTTCTG gttagt C -3.5 tattgctttcag ACTGGCTG IleSerA spTrpLeu 664 4 255 GTCCAAG gtgagc D -2.1 cctcocttccag ATCCAGCA ValGinA spProAla 1312 5 648 TCCTATAATAAA Exon sequences are shown in upper case letters; intron sequences are shown in lower case letters. Nucleotides bordering introns are numbered according to ref. 3. The predicted site of peptidase cleavage is indicated by an arrow. *Size of the longest known transcribed portion of exon 1 starting with nucleotide 1 (3). Downloaded by guest on September 30, 2021 2858 Immunology: Kulczycki et al. Proc. Natl. Acad Sci. USA 87 (1990) lkb I4-1J // Z

B RHB R (R) H B 'C1 IEC TM-4i---A C1 C2 3 -9 4 Kb 1* ECI EC2 TMA Cl C2 C3

A 4 b clone 1-1 clone 4-5 clone 5-1

FIG. 3. Partial map ofthe mouse FcYRIIb gene. Exons are shown as boxes. L* indicates a 21-bp exon that encodes a portion of the leader peptide, EC1 and EC2 denote exons coding for immuno- globulin-like extracellular domains, TM indicates an as yet uncloned presumed exon encoding the transmembrane region, and C1, C2, and C3 represent exons coding from cytoplasmic regions of the receptor. Restriction enzyme sites for BamHI (B), EcoRI (R), and HindIlI (H) are indicated. The three subclones used for mapping and sequencing are labeled. Only the translated portion of exon C3 is shown. ern blots (Fig. 4 and ref. 4) could be accounted for by the FcYRIIa and FclRIIb gene maps. This suggests that these loci exist as single copies in the mouse genome. FIG. 4. Southern blot analysis of FcYRII genes. C57BL/6 mouse Linkage of FcRlla and Genes. By using CHEF liver DNA was digested with EcoRI (lane 1) or HindIII (lane 2) and FcyRIIb probed with the 1.5-kb insert of clone b (FcYR1Ib cDNA). Molecular gels and FcRRIIa-specific and FcYRIIb-specific probes, both size markers are indicated. The Fc7RIIa gene accounts for the of the genes were localized to the same Nar I fragment and 10.1-kb EcoRI band (exons 2-5) and the 6.0-kb HindIII band (exons Sfi I partial digest fragment, each "200 kb in size, and to the 3-4) of the genomic Southern blot. The FcYRIIb gene structure same ""160-kb Nar I/Sfi I fragment (data not shown). Spec- accounts for the 1.4-kb EcoRI band (exon EC1), and the remaining ificity is established because these genes were localized to '3-kb and '9-kb EcoRI bands presumably represent the fragment different Xho I fragments. containing the EC2 exon and the fragment containing the cytoplasmic exons, respectively. DISCUSSION (255 bp) (97.6% nucleotide identity). This marked homology and Putative Protein between extracellular domains is consistent with the obser- Gene Organization Domains. The vations that both receptors demonstrate similar specificities structure and organization ofthe mouse and Fc RlIb Fc'YRIla in binding to Fc regions of IgG (6). In contrast, the mem- genes have been investigated in the present study. The brane-spanning and intracytoplasmic domains of these re- gene consists of five exons contained within 9 kb FcYRIIa ceptors lack homology (3). Furthermore, the portions of the (Fig. 1 and Table 1). The Fc',RIIb gene has a more complex genes these domains are markedly different in organization that includes at least eight exons (Fig. 3 and encoding one exon Table 2). Exons 3 and 4 of the FczRIIa gene and the organization-the FczRIIa gene utilizes only (exon corresponding exons of the FcRIIb gene (EC1 and EC2) 5) to encode the entire transmembrane-cytoplasmic region, encode the extracellular domains of 85 or 86 amino acids, whereas the FcyRIIb gene uses at least four exons. In this which are homologous to other members ofthe immunoglob- region the lack of homology and the differences in genomic ulin supergene family. Each extracellular domain of both Fc structure suggest that these receptors may transduce signals receptor genes is encoded by a separate exon, which is a and function intracellularly in very different ways. common characteristic of this supergene family (26). The The genomic organization of the mouse FcYRIIa gene is nucleotide homology between Fc,,RIIa gene exon 3 (258 bp) strikingly similar to the organization of the rat Fc6RI a-chain and the corresponding Fc,,RIIb gene EC1 exon (255 bp) is gene, which contains five analogous exons (28). Also, it has 95.7%. Also, the FcYRIIa gene exon 4 (255 bp) is extremely been noted that mouse FcYRIIa and rat Fc6RI a-chain share homologous to the corresponding FcrRIIb gene EC2 exon an identical eight-amino acid transmembrane sequence (25) Table 2. Partial exon-intron organization of the mouse FcRIIb gene Exon Intron Designation Size, bp 5' splice donor size, kb 3' splice acceptor 412 (L') (-) GTGCTAA cctttcttacag ATCTTGCT 433 L* 21 |ACTCATG gtaagt "1.2 aaaattgagcag ATCTTCCA 688 EC1 255 ATTTCTG gttagt %1.4 ctttcag ATCTTCCA 943 - EC2 255 GTCCAAG gtgagc - GGCCCAAG 1066 (TM) (123) GTTCCAG - tgcccctcctag CTCTCCC 1207 C1 141 AGCCCAT gtgagt 0.75 tttcatccacag ACAATCCT 1245 C2 38 ACTGAG gtgagg 0.10 tgotttccctag GCTGAG 1332 C3 ATTTAG Exon sequences are shown in upper case letters; intron sequences are shown in lower case letters. Nucleotides bordering introns are numbered according to Ravetch et al. (3) as amended (4). Parentheses and dashes indicate gaps in genomic organization. Only the translated portion of exon C3 is shown. The predicted site of peptidase cleavage (3-5) is indicated by an arrow. Downloaded by guest on September 30, 2021 Immunology: Kulczycki et al. Proc. Natl. Acad. Sci. USA 87 (1990) 2859 and associate with the same Mr 10,000 subunit (29). Together, Note Added in Proof. We have found transcriptional startpoints ofthe these observations suggest that FcYRIIa and FceRI a chain FcRIIa gene to be at nucleotides 1 and 37 (Table 1) by using RNase (but not FcRIIb) might share functional similarities or asso- protection and S1 nuclease assays. Regarding the biological conse- ciate with similar components. Like the FcYRIIa and FcRI quences of alternative mRNA splicing events, it has recently been a-chain genes, the genes encoding the T-cell receptor a chain, shown that the C1 exon of the FcRIIb gene encodes a region that prevents accumulation of the receptor in clathrin-coated pits (37). the major histocompatibility complex class II a chain, and the Also, analogous to the 21 nucleotide exons that we describe (Fig. 2), MRC OX-2 glycoproteins utilize only one exon to encode their the human FcyRIIb gene contains a 21-nucleotide exon, and a entire transmembrane and cytoplasmic region (26). transcript lacking it has been found (38). We suggest that alternative As a rule, members of the immunoglobulin supergene splicing that deletes transcription of this 21-nucleotide "mini-exon" family encode their signal sequences by using a single exon may produce "proreceptors" that do not undergo peptidase cleavage (26). The first exceptions to this rule were the mouse and but have N termini tethered to the membrane until an alternate human CD3 y chains, which utilize one exon to encode the proteolytic event. to hydrophobic region of the leader peptide and another We thank Evan Sadler for synthesis of oligonucleotides, Mark encode the signal peptidase cleavage site (30, 31). The FcRI Behlke for the NZW cDNA libraries, and Sarah K. Bronson and Jean a-chain gene (28) and the FcYRIIa and FcRIIb genes now Molleston for assistance with CHEF gels. This work was supported represent additional exceptions in the immunoglobulin gene by the National Institutes of Health (Al 24005). superfamily to the "signal peptide-single exon" rule. These three related genes each contain an unusually small con- 1. Unkeless, J. C., Scigliano, E. & Freedman, V. H. (1988) Annu. served exon (21 bp) that encodes the amino acids ofthe leader Rev. Immunol. 6, 251-281. 2. Kinet, J.-P. (1989) Cell 57, 351-354. peptide that are predicted to form part of the peptidase- 3. Ravetch, J. V., Luster, A. D., Weinshank, R., Kochan, J., recognition site and to contain or to border the peptidase Pavlovec, A., Portnoy, D. A., Hulmes, J., Pan, Y-C. E. & cleavage site (Fig. 2). Based on statistical prediction methods Unkeless, J. C. (1986) Science 234, 718-725. (23, 24), we predict that peptidase cleavage of FcYRIIa occurs 4. Hogarth, P. M., Hibbs, M. L., Bonadonna, L., Scott, B. M., between Ala-31 and Leu-32 (Fig. 2), rather than the site Witort, E., Pietersz, G. A. & McKenzie, I. F. C. (1987) Im- previously predicted (3). Thus, in the FcYRIIa gene, the first munogenetics 26, 161-168. three exons are predicted to encode the signal sequence. The 5. Lewis, V. A., Koch, T., Plutner, H. & Mellman, I. (1986) 21-bp exons of the three Fc receptor genes exhibit consid- Nature (London) 324, 372-375. erable nucleotide homology, but little amino acid homology, 6. Weinshank, R. L., Luster, A. D. & Ravetch, J. V. (1988) J. Exp. Med. 167, 1909-1925. and different predicted peptidase cleavage sites (Fig. 2). 7. Heusser, C. H., Anderson, C. L. & Grey, H. M. (1977) J. Exp. The exon-intron boundaries of the FcYRIIa and FcYRIIb Med. 145, 1316-1327. genes are quite similar. For example, the 5' splice donor sites 8. Segal, D. M. & Titus, J. A. (1978) J. Immunol. 120, 1395-1403. of FcYRIIa gene introns B, C, and D (Table 1) are identical 9. Holmes, K. L., Palfree, R. G. E., Hammerling, U. & Morse, to the homologous donor sites of the FcYRIIb gene (Table 2). H. C. (1985) Proc. Natl. Acad. Sci. USA 82, 7706-7710. It has been recognized that mouse FcRIIa and FcRIIb 10. Hibbs, M. L., Hogarth, P. M. & McKenzie, I. F. C. (1985) are homologous to human FcYRII (32-34) and human FcYRIII Immunogenetics 22, 335-348. (35, 36), particularly in their extracellular domains. Compar- 11. Festenstein, H., Bishop, C. & Taylor, B. A. (1977) Immuno- ison of mouse gene exons 2, 3, and 4 with the genetics 5, 357-361. FcYRIIa 12. Kappler, J. W., Staerz, U., White, J. & Marrack, P. C. (1988) corresponding regions of human FcYRII gene (nucleotides Nature (London) 332, 35-40. 91-111, 112-369, and 370-624 of ref. 33) shows 76%, 72%, 13. MacDonald, H. R., Schneider, R., Lees, R. K., Howe, R. C., and 73% nucleotide homology, respectively. Although exon Acha-Orbea, H., Festenstein, H., Zinkernagel, R. M. & Hen- 5 of FcYRIIa gene has little homology, the C2 and presumed gartner, H. (1988) Nature (London) 332, 40-45. TM exons ofFcYRIIb gene (Fig. 3) demonstrate 66% and 71% 14. Feinberg, A. P. & Vogelstein, B. (1983) Anal. Biochem. 132, homology, respectively, with corresponding regions of hu- 6-13. man FcYRII gene (32-34). Also, the ends ofthe human FcYRII 15. Grosveld, F. G., Lund, T., Murray, E. J., Mellor, A. L., Dahl, intron "remnant," CTTTCTGgtcagt.. .tgtgtctttcagAGTG- H. H. M. &Flavel, R. A. (1982) NucleicAcidsRes. 10,6715-6732. GCTG (34), where lower case letters signify intron se- 16. Maniatis, T., Fritsch, E. F. & Sambrook, J. (1982) Molecular Cloning: A Laboratory Manual (Cold Spring Harbor Lab., Cold quences, are remarkably similar to the corresponding mouse Spring Harbor, NY). FcYRIIa intron (intron C of Table 1), which suggests that 17. DiLella, A. G. & Woo, S. L. C. (1987) Methods Enzymol. 152, exon-intron boundaries may also be conserved across spe- 199-212. cies in this gene family. 18. Chaplin, D. D., Woods, D. E., Whitehead, A. S., Goldberger, Evolution and Linkage of Fc,,R Genes. The similarities in G., Colten, H. R. & Seidman, J. G. (1983) Proc. Natl. Acad. exon structures and the 96-98% nucleotide homology be- Sci. USA 80, 6947-6951. tween exons encoding extracellular domains indicate that the 19. Smith, C. L. & Cantor, C. R. (1987) Methods Enzymol. 155, FcRIIa and FcYRIIb genes are products ofgene duplication. 449-467. This conclusion is supported by the demonstration that both 20. Chu, G., Vollrath, D. & Davis, R. W. (1986) Science 134, 160-kb genomic DNA 1582-1585. genes are physically linked on the same 21. Breathnach, R. & Chambon, P. (1981) Annu. Rev. Biochem. 50, fragment. The duplication of the two extracellular exons 349-383. within each gene must have occurred much earlier than the 22. Mount, S. M. (1982) Nucleic Acids Res. 10, 459-472. duplication to form two genes because the nucleotide homol- 23. von Heijne, G. (1983) Eur. J. Biochem. 133, 17-21. ogy between exons 3 and 4 of the FcRIIa gene is 44.2% and 24. von Heijne, G. (1986) Nucleic Acids Res. 14, 4683-4690. homology between FcYRIIb exons EC1 and EC2 is 49.3%. 25. Kinet, J.-P., Metzger, H., Hakimi, J. & Kochan, J. (1987) The extraordinary homology between genes suggests that Biochemistry 26, 4605-4609. gene duplication has been a relatively recent event and/or 26. Williams, A. F. & Barclay, A. N. (1988) Annu. Rev. Immunol. that strong selection pressures have been exerted on these 6, 381-405. to these structures. 27. Proudfoot, N. J. & Brownlee, G. G. (1976) Nature (London) receptors conserve 263, 211-214. The close proximity of these genes should facilitate map- 28. Tepler, I., Shimizu, A. & Leder, P. (1989) J. Biol. Chem. 264, ping of this important region of mouse chromosome 1. 5912-5915. Mapping may be useful in localizing the gene locus that 29. Ra, C., Jouvin, M.-H. E., Blank, U. & Kinet, J.-P. (1989) encodes the Mls-Ja product, which has been inseparable from Nature (London) 341, 752-754. the FcYRII genes in genetic studies (9-11). 30. Saito, H., Koyama, T., Georgopoulos, K., Clevers, H., Haser, Downloaded by guest on September 30, 2021 2860 Immunology: Kulczycki et al. Proc. Nati. Acad. Sci. USA 87 (1990)

W. G., LeBien, T., Tonegawa, S. & Terhorst, C. (1987) Proc. 34. Stengelin, S., Stamenkovic, I. & Seed, B. (1988) EMBO J. 7, Nadl. Acad. Sci. USA 84, 9131-9134. 1053-1059. 31. Tunnacliffe, A., Buluwela, L. & Rabbitts, T. H. (1987) EMBO 35. Simmons, D. & Seed, B. (1988) Nature (London) 333, 568-570. J. 6, 2953-2957. 36. Peltz, G. A., Grundy, H. O., Lebo, R. V., Yssel, H., Barsh, 32. Stuart, S. G., Trounstine, M. L., Vaux, D. J., Koch, T., Mar- G. S. & Moore, K. W. (1989) Proc. Natl. Acad. Sci. USA 86, tens, C. L., Mellman, I. & Moore, K. W. (1987) J. Exp. Med. 1013-1017. 166, 1668-1684. 37. Miettinen, H. M., Rose, J. K. & Mellman, I. (1989) Cell 58, 33. Hibbs, M. L., Bonadonna, L., Scott, B. M., McKenzie, 317-327. I. F. C. & Hogarth, P. M. (1988) Proc. Nadl. Acad. Sci. USA 38. Brooks, D. G., Qiu, W. Q., Luster, A. D. & Ravetch, J. V. 85, 2240-2244. (1989) J. Exp. Med. 170, 1369-1385. Downloaded by guest on September 30, 2021