CARMIL family proteins as multidomain regulators of actin-based motility

Abstract

CARMILs are large multidomain proteins that regulate the actin-binding activity of capping protein (CP), a major capper of actin filament barbed ends in cells. CARMILs bind directly to CP and induce a conformational change that allosterically decreases but does not abolish its actin-capping activity. The CP-binding domain of CARMIL consists of the CP-interaction (CPI) and CARMIL-specific interaction (CSI) motifs, which are arranged in tandem. Many cellular functions of CARMILs require the interaction with CP; however, a more surprising result is that the cellular function of CP in cells appears to require binding to a CARMIL or another protein with a CPI motif, suggesting that CPI-motif proteins target CP and modulate its actin-capping activity. Vertebrates have three highly conserved genes and expressed isoforms of CARMIL with distinct and overlapping localizations and functions in cells. Various domains of these CARMIL isoforms interact with plasma membranes, vimentin intermediate filaments, SH3-containing class I myosins, the dual-GEF Trio, and other adaptors and signaling molecules. These biochemical properties suggest that CARMILs play a variety of membrane-associated functions related to actin assembly and signaling. CARMIL mutations and variants have been implicated in several human diseases. We focus on roles for CARMILs in signaling in addition to their function as regulators of CP and actin.

Comparative analysis of CPI-motif regulation of biochemical functions of actin capping protein

“…One cellular pool of CP with potentially great physiological significance is that found as a 1:1 complex with a CPI-motif protein that has been targeted to a membrane (11,14). Cell studies suggest that this pool of CP is part of the active fraction of CP, available to cap barbed ends of actin filaments that are near membranes (15,34).…”

“…Cell cytoplasm contains relatively high concentrations of V-1 and CP, and both proteins appear to diffuse freely about the cell (9,10). In contrast, CPI-motif proteins are present in far lower amounts, and they are generally targeted to membranes (11)(12)(13). Proteins with CPI motifs are otherwise unrelated to each other, and cells may express one or more CPI-motif proteins (reviewed in (11,14)).…”

“…In contrast, CPI-motif proteins are present in far lower amounts, and they are generally targeted to membranes (11)(12)(13). Proteins with CPI motifs are otherwise unrelated to each other, and cells may express one or more CPI-motif proteins (reviewed in (11,14)). The different proteins are generally targeted to membranes by other domains of the proteins, where they are proposed to promote actin polymerization and actin-based motility (11,14).…”

“…First, V-1 has been shown to inhibit CP in cells (9). Second, a number of CPI-motif proteins have been found to require interaction with CP for their cellular activity (reviewed in (11,14)). Third, mutations of CP that inhibit the binding of CPI-motif proteins lead to an apparent loss of function of CP in cells (15).…”

“…To investigate the biochemical basis for how the different types of CPI-motif proteins may function in cells, we examined the interactions among CP, actin barbed ends, V-1 and CPImotifs, using purified components in vitro. Many of these interactions have been studied in previous work, in studies of one or more of these components and one or more of the various CPI-motif proteins (1,2,7,8,11,14,(16)(17)(18)(19). Our goal here was to provide a comparative analysis of the different CPI-motif proteins, one with another, and examine how their effects on actin capping and V-1 binding correlate with each other and with their affinity for binding CP.…”