456 TIBS - November 1985 microvilli (see below), etc. Trans- Microfilament-membrane membrane interactions which involve cytoskeletal elements are commonly interaction found in adherens and desmosomal junctions, in surface patches and caps Benjamin Geiger induced by multivalent ligands, etc. (see Ref. 1 and discussion below). Surveys of a large variety of biological One of the major topics of research in modern cell biology involves the structure and systems suggested that the induction of organization of the cytoskeleton and, in particular, the interaction of its various surface specialization due to the interac- components with the plasma membrane. Studies carried out in many laboratories over tion between the cytoskeleton and the the last several years pointed to the complex and heterogeneous nature of membrane- inner surfaces of the plasma membranes cytoskeleton interactions in different systems. This is manifested by a remarkable cell- can play multiple roles of great physio- type'specificity as well as distinct differences between the mode of anchorage of the logical significance in processes such as various cytoskeletal components. Moreover the assembly and maintenance of each of cell motility, attachment, division and the cytoskeletal systems within individual cells appears to be a dynamic process which recognition. is probably spatially and temporally regulated and modulated. Microfilaments are associated with the Here, examples are presented to illus- and topology of the latter. Many studies, plasma membrane via specific linker trate some of the common features of both biophysical and structural, have proteins the interactions which occur at the mem- suggested that membrane constituents Microfilaments constitute one of the brane-microfilament interphase, and may sometimes display non-random dis- major cytoskeletal systems in living cells4 their physiological significance is dis- tribution, forming specialized domains cussed (for a recent review see Ref. 1). with distinct topology, structure and One of the most important con- functions. Many such domains have sequences of membrane-microfilament been identified, for example, the synap- interaction is the local perturbation of tic clusters of acetyicholine receptors, the membrane and the outcoming alter- receptors to low-density lipoprotein, ations in its biochemical and biophysical LDL (or to other ligands) which are pre- properties. The generation of molecular clustered in coated pits, surface microdomains in biological membranes glycoproteins in budding enveloped by such mechanisms has central roles in viruses, and specific cellular junctions. cellular physiology, as discussed below. Moreover, morphological specializa- tions and irregularities of the free Microdomains in membranes: the surface could often be seen by transmis- involvement of the cytoskeleton sion or scanning electronmicroscopy 3. The widely accepted fluid mosaic In principle, there are several possible model depicts biological membranes as mechanisms for microdomain forma- two-dimensional lipid bilayers in which tion, some of which are outlined in 'integral proteins' are embedded2. It was Fig. 1. proposed that these membrane proteins The starting state is the cell depicted are free to laterally diffuse through the on the left of Fig. 1 with different types fluid phase and thus reach a uniform of homogeneously distributed surface distribution. It was, nevertheless, molecules (the squares and circles). This pointed out in the original formulation cell might undergo various changes of the model that the lateral mobility of which could lead to the segregation of the integral membrane constituents may the 'squares' and 'circles' into specific, be restricted by interactions with specialized and mutually exclusive Fig. 1. Four different mechanisms for the genera- immobile matrices at the cytoplasmic or microdomains (see caption to Fig. 1). tion of non-homogenous distribution of integral extracellular interphases of the plasma The mechanisms shown in Figs lc and d membrane proteins. The cell on the left has its membrane. Such interactions may be are of direct interest to the topic of this 'square' and "circular' receptors homogenously dis- article; they may involve either a uni- tributed. (a) Formation of a cluster of the 'square' mediated by 'peripheral membrane pro- receptors by intramembranal interactions. This teins', namely proteins which are tightly lateral association of the membrane aggregation may be triggered by interaction with bound to either surface of the mem- receptors with intracellular matrices, another membrane protein, modification of the brane without being actually inserted namely the cytoskeleton (c) or a trans- "squares', changes in medium conditions (pH, into its bilayer. Furthermore, it was con- membrane interaction with insoluble ionic environment), etc. (b) Exofacial interactions ceivable that if the peripheral elements elements at the two faces of the plasma- with extracellular matrices leading to immmobiliza- lemma (d). Examples of the former tion and aggregation of a particular class of recep- form mechanically stable networks, tors. (c) Endofacial interactions, namely their attachments to the membrane may might be the association of band-3 microdomain formation induced by an interaction have long-range effects on the dynamics protein with the cytoskeletal shell in of the 'square" integral proteins with cytoskeletal erythrocytes which is mediated by filaments. (d) Transmembrane interactions involv- B. Geiger is at the Department of Chemical Immu- ing an integral membrane element attached to both nology, The Weizmann Institute of Science, ankyrin, the attachment of microfila- extracellular and intracellular matrices. Rehovot 76100, Israel. ments to the membrane in epithelial t~) 1985, Elsevier Science Publishers B.V., Amsterdam 0376 - 5(~67/85/$02.(D TIBS - November 1985 457 and are present predominantly in the As pointed out, a more widely the linkage between F-actin and the cortical cytoplasm, subjacent to the plas- accepted mechanism for the anchorage membrane in intestinal microvilli. The malemma. Many biochemical and ultra- of actin may involve peripheral associa- advantage of this system which attracted structural studies indicated that tion with the cytoplasmic faces of the the attention of Mooseker and Tilney microfilaments are not only enriched membrane, mediated by specific integral over a decade ago 6, lies in its relatively near the plasma membrane but are often receptor(s) and peripheral linking- regular structure and the presence of actually attached to its endofacial sur- protein(s). only a few cytoskeletal components7. faces (see Ref. 1). The cell types and An excellent example which may help Electronmicroscope analyses of intesti- subcellular assemblies in which mem- to illustrate this mode of anchorage is nal epithelium indicated that a bundle of brane-microfilament interactions occur are however quite diverse in their origins and molecular properties. They include sites such as microvilli of polar- ized epithelia, lamellipodia and ruffling membranes, stereocilia of the sensory epithelium in the cochlea, contractile ring in dividing cells, end-on and lateral connections between myofibrils and the sarcolemma, dense plaques of smooth muscle, and intercellular junctions of the adherens type in a large variety of cells. In these locations, membrane-microfila- ment interaction probably affects such cardinal processes as cell morpho- genesis, sensation, mobility, division, force generation, intercellular adhesion and cell communication. The involve- ment of membrane-microfilament inter- action in such an important battery of cellular activities has motivated a gen- eral search for ubiquitous linkers of actin to the membrane. However, as one might have antici- pated on the basis of the apparent struc- tural variability, direct biochemical and ¢. immunochemical data revealed con- b siderable heterogeneity in the molecular mechanisms of membrane anchorage in the different systems. One of the com- mon features, however, was the pres- ence of specific linker proteins capable: of bridging between F-actin and the membrane5. The necessity of accessory proteins for membrane anchorage has raised some controversy since the pos-. sibility has been considered over the', years that actin filaments may be', directly inserted into the lipid domain of the plasmalemma. This suggestion wa,; supported by occasional reports on the: presence of 'cell surface actin' or on the: effects of anti-actin antibodies on the physiology of living cells. This view, however, is usually regarded as unlikely or, at least, physiologically irrelevant; spontaneous penetration of a hydro- philic protein such as actin into the core of the membrane seems to be extremely unfavorable thermodynamically and the incidental immunocytochemical detec- tion of actin on the outer cell surface Fig. 2. The structure and molecular organization of intestinal microvilli. (a) and (b): Transmission could be attributed, in most cases, to an electronmicrographs of cross-sections (a) and longitudinal sections (b) through chicken intestinal micro- artefactual binding of actin released villi. (c) Scheme depicting the locations of the different cytoskeletal proteins of microvilli: F, fimbrin; V, from dead cells. villin; 110 kDa protein; C, calmodulin. The nature of the electron-dense material of the tip is not known. 458 TIBS - November 1985 microfilaments runs from the tip down villi belong to the third category of molecular