CARMIL or LRRC16A

CARMIL (protein CP ARp2/3 myosin I linker) was initially named Acan125 when it was discovered in amoeba where it was found to bind the SH3 domain of class I myosin molecules. Subsequently, this protein was found to bind capping protein (CP) and to Arp2/3 complex which to it being named as CARMIL. This is a large protein and has a long leucine-rich repeat (LRR) region. The function of this LRR region is unclear (PMID: 18544499).

CARMIL is important in actin-based cell motility. In addition to its role in many cellular process, actin is also involved in cell motility and shape change. Actin polymerization occurs mainly at the barbed ends of the actin filament. Addition of new actin molecules to the barbed ends of actin filaments are thought to be responsible for cell motility and change in shape. The assembly of actin filament by addition of actin monomers to the barbed ends is inhibited by Capping Proteins (CP). These capping proteins bind and thus hide the barbed ends of the actin filaments making them inaccessible to other actin molecules. CP is  a heterodimer of two subunits with a shape similar to a mushroom. CARMIL binds to the CP and this binding of CARMIL to CP results in decreased affinity of the CP to barbed ends of actin filament and dislodges itself from it. Thus barbed ends are exposed and actin polymerization starts resulting in cell-shape (PMID: 16434392).

Of note, the ability of CARMIL to expose barbed ends of actin filament is specific to CP and doesn’t include other proteins that inhibit actin polymerization. The CP binding region of CARMIL resides in the later part of the protein called CAH3 region (CARMIL homology domain 3; amino acid residues 940-1121). Within this region, a portion of 25 amino acids that is highly conserved from protozoa to flies, to worms to vertebrates. Point mutations in this region result in loss of CP-binding ability of CARMIL (PMID: 16434392).

A patch consisting of basic amino acids on CP is essential for its interaction with barbed ends of the actin. Using nuclear magnetic resonance (NMR), Zwolak et al  studied the interaction of the CAH3 domain and CP in a mouse model. They found that the highly basic mouse CAH3a subdomain binds with high affinity to a complementary “acidic groove” on CP.  This CAH3a-CP interaction orients the CAH3b subdomain directly adjacent to the basic patch of CP. The importance of specific residue interactions between CP and CAH3a/b was confirmed by site-directed mutagenesis of both proteins (PMID: 20630878).

In another study Hernandez-Valladares identified two similar regions  within the CAH3(965–1038) performing essentially the same two functions. The first region, Ile971–Cys1004, is CP interacting motif. The second region, Arg1021–Thr1035, binds to the underside of the CP mushroom cap on the opposite side of the CP mushroom stalk to which the CP interaction motif binds. Together, these two regions resemble a finger (CPI) and thumb (CSI) encircling the stalk on the underside of the mushroom cap (PMID: 20357771).

LRRC16A or CARMIL gene is located on chromosome 6p22.2, has 341,453 base-pairs, and 36 exons. As is also clear from the LD block of YRI (data from HapMap, software used Haploview) the later part of the gene is conserved and is present in a larger LD block.