International Journal of Molecular Sciences Review Structure, Function, and Regulation of the SRMS Tyrosine Kinase Chakia J. McClendon 1 and W. Todd Miller 1,2,* 1 Department of Physiology and Biophysics, Stony Brook University, Stony Brook, NY 11794-8661, USA; [email protected] 2 Department of Veterans Affairs Medical Center, Northport, NY 11768, USA * Correspondence: [email protected] Received: 21 May 2020; Accepted: 12 June 2020; Published: 14 June 2020 Abstract: Src-related kinase lacking C-terminal regulatory tyrosine and N-terminal myristoylation sites (SRMS) is a tyrosine kinase that was discovered in 1994. It is a member of a family of nonreceptor tyrosine kinases that also includes Brk (PTK6) and Frk. Compared with other tyrosine kinases, there is relatively little information about the structure, function, and regulation of SRMS. In this review, we summarize the current state of knowledge regarding SRMS, including recent results aimed at identifying downstream signaling partners. We also present a structural model for the enzyme and discuss the potential involvement of SRMS in cancer cell signaling. Keywords: tyrosine kinase; signal transduction; phosphorylation; SH3 domains; SH2 domains; Src family kinases 1. Introduction Src-related kinase lacking C-terminal regulatory tyrosine and N-terminal myristoylation sites (SRMS) is a 53 kDa nonreceptor tyrosine kinase that was first discovered in mouse neural precursor cells [1]. Studies of the expression pattern of SRMS mRNA showed that the highest levels were present in lung, testes, and liver tissues. Full-length cDNA encoding SRMS was isolated from a murine lung cDNA library, with the predicted SRMS protein containing all of the conserved amino acid residues expected for a nonreceptor tyrosine kinase. Targeted disruption of the gene encoding SRMS did not lead to any apparent phenotype, suggesting that the kinase may have functions that are redundant in regard to other tyrosine kinases [1]. A few years later, the SRMS gene (designated PTK70) was isolated from a neonatal murine skin library and was found to be most strongly expressed in normal epidermal and keratinocyte cells [2]. While SRMS was initially considered to be a member of the Src family of nonreceptor tyrosine kinases, analyses of the amino acid sequence and intron/exon structure placed SRMS in a separate family, the Brk family [3,4]. This group of nonreceptor tyrosine kinases also includes the Fyn-related kinase (Frk) and breast tumor kinase (Brk; also called PTK6) [3–5]. The human SRMS gene maps to chromosome 20q13.3, which is only 1.5 kbp away from the Brk gene, suggesting that the two genes may interact genetically [4,6]. The two genes also map together on chromosome 2 in mice [1,4]. Indeed, the two protein kinases interact biochemically; SRMS phosphorylates the C-terminus of Brk [7]. This portion of human chromosome 20 is a region of the genome that was found to be amplified in breast cancers [8]. SRMS, like Brk, is highly expressed in most breast carcinoma cells compared to normal mammary cell lines and tissues [9]. The goal of this review is to summarize past and recent findings regarding SRMS. Compared to other nonreceptor tyrosine kinases, there is very little information available regarding the structure, function, and regulation of SRMS. Herein, we review what is known about the substrates and downstream Int. J. Mol. Sci. 2020, 21, 4233; doi:10.3390/ijms21124233 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2020, 21, x FOR PEER REVIEW 2 of 13 Int. J. Mol. Sci. 2020, 21, 4233 2 of 13 downstream signaling partners of SRMS. Wherever possible, we compare SRMS to the well- characterized nonreceptor tyrosine kinase Src. Our goal is to provide a foundation for future studies signaling partners of SRMS. Wherever possible, we compare SRMS to the well-characterized nonreceptor in the field. tyrosine kinase Src. Our goal is to provide a foundation for future studies in the field. 2.2. Structure Structure and and Activity Activity of of SRMS SRMS 2.1.2.1. Domain Domain A Architecturerchitecture SRMSSRMS is is a a 488 488-amino-amino acid acid nonreceptor nonreceptor tyrosine tyrosine kinase kinase containing containing three three functional functional domains domains that that waswas first first identified identified in Src family kinases (SFKs). From From N N-terminus-terminus to to C C-terminus,-terminus, the thesese are are the the Src Src homologyhomology 3 (SH3),(SH3), SrcSrc homology homology 2 (SH2),2 (SH2), and and kinase kinase catalytic catalytic domains domains [1,3 ][1,3] (Figure (Figure1). Within 1). Within the SFKs, the SFKs,this domain this domain architecture architecture is critical is for critical mediating for mediating inter- and intramolecularinter- and intramolecular interactions, downstreaminteractions, downstreamsignaling, and signaling, regulation and [10 regulation–13]. The [10 SH3–SH2-kinase–13]. The SH3– domainSH2-kinase arrangement domain arrangeme is found innt otheris found Brk infamily other kinases, Brk family as. well kinases as in, theas well Src, Abl,as in Tec, the andSrc, Csk Abl, families Tec, and of nonreceptorCsk families tyrosineof nonreceptor kinases, tyrosine and was kinases,conserved and from was unicellular conserved eukaryotes from unicellular to humans eukaryotes [14–16]. Brk to familyhumans kinases [14–16] appear. Brk to famil be absenty kinases from appearthe genomes to be absent of premetazoans. from the genomes Brk-like of kinases premetazoans. are found Brk in -Amphioxuslike kinasesand are the found cnidarian in AmphioxusNematostella and thevectensis cnidarian, suggesting Nematostella that an vectensis SFK/Brk, suggesting gene in a metazoan that an SFK/Brk ancestor wasgene duplicated, in a metazoan giving ancestor rise to SFKswas duplicatedand Brk-family, giving kinases rise to [17 SFKs]. SRMS and Brk itself-family is present kinases only [17] in. vertebrates.SRMS itself is present only in vertebrates. FigureFigure 1. 1. DomainDomain structure structuress of of Src Src kinase kinase and and SRMS SRMS (S (Src-relatedrc-related kinase kinase lacking lacking C C-terminal-terminal r regulatoryegulatory tyrosinetyrosine and N-terminalN-terminal myristoylationmyristoylation sites). sites). The The major major autophosphorylation autophosphorylation sites sites are are indicated indicated in red. in red.In Src, In phosphorylationSrc, phosphorylation of Y527 of stabilizesY527 stabilizes the autoinhibited the autoinhibited conformation. conformation. SRMS lacksSRMS the lacks N-terminal the N- terminalmyristoylation myristoylation site found site in found Src. in Src. SrcSrc contains contains a a 14 14-carbon-carbon bound bound myristoyl myristoyl group group on on the the N N-terminus-terminus that that is is capable capable of of interacting interacting withwith the the inner inner leaflet leaflet of of the the plasma plasma membrane membrane [10,18 [10–,1820]–.20 Protein]. Protein N-myristoylation N-myristoylation is a co is- and a co- post and- translationalpost-translational modification modification that thatrequires requires removal removal of ofthe the N- N-terminalterminal Met Met residue, residue, and and enzymatic enzymatic myristoylmyristoyl transfer transfer to to a a conserved conserved Gly Gly residue. residue. Myristoylation Myristoylation of of Src Src allows allows the the kinase kinase to to interact interact with with aa variety variety of of extracellular receptors receptors and and to to activate a a variety of downstream signaling pathways pathways that that triggertrigger cell cell transformation transformation [19,21] [19,21].. Myristoylation Myristoylation is is important important for for the the role role of of Src Src in in prostate prostate c cancerancer developmentdevelopment [22] [22].. Unlike Unlike Src, Src, SRMS SRMS lacks lacks the the myristoylation myristoylation site site and and membrane membrane anchoring anchoring signals signals (Figure(Figure 11).). Therefore,Therefore, SRMSSRMS isis anan intracellularintracellular proteinprotein (discussed(discussed inin moremore detaildetail below).below). TheThe aminoamino terminalterminal region region of of SRMS SRMS contains contains a a 51 51-amino-amino acid acid sequence sequence preceding preceding the SH3 SH3 domain. domain. In In Src Src family family kinases,kinases, this regionregion isis unique unique to to each each kinase kinase and and is involvedis involved in specificin specific protein–protein protein–protein interactions interactions with withsignaling signaling partners partners [23]. [23] NMR. NMR and and small-angle small-angle x-ray x-ray scattering scattering (SAXS) (SAXS) experiments experiments showed showed that th anat anintrinsically intrinsically disordered disordered region region within within the c-Src the uniquec-Src unique domain domain forms aforms fuzzy a intramolecular fuzzy intramolecular complex complexinvolving involving the SH3 domain the SH3 [24 domain]. For SRMS, [24]. For removal SRMS, of removal the N-terminal of the regionN-terminal results region in a complete results in loss a completeof kinase loss activity of kinase in transfected activity in mammalian transfected cells, mammalian indicating cells, that indicating the N-terminus that the plays N-terminu an importants plays anrole important in maintaining role in kinase maintaining activity kinase [9]. The activity N-terminal [9]. The portion N-terminal of the SRMSportion unique of the domain SRMS unique has the domainpotential has to formthe potential an amphipathic