Capping protein regulatory cycle driven by CARMIL and V-1 may promote actin network assembly at protruding edges.
Although capping protein (CP) terminates actin filament elongation, it promotes Arp2/3-dependent actin network assembly and accelerates actin-based motility both in vitro and in vivo. In vitro, capping protein Arp2/3 myosin I linker (CARMIL) antagonizes CP by reducing its affinity for the barbed end and by uncapping CP-capped filaments, whereas the protein V-1/myotrophin sequesters CP in an inactive complex. Previous work showed that CARMIL can readily retrieve CP from the CP:V-1 complex, thereby converting inactive CP into a version with moderate affinity for the barbed end. Here we further clarify the mechanism of this exchange reaction, and we demonstrate that the CP:CARMIL complex created by complex exchange slows the rate of barbed-end elongation by rapidly associating with, and dissociating from, the barbed end. Importantly, the cellular concentrations of V-1 and CP determined here argue that most CP is sequestered by V-1 at steady state in vivo. Finally, we show that CARMIL is recruited to the plasma membrane and only at cell edges undergoing active protrusion. Assuming that CARMIL is active only at this location, our data argue that a large pool of freely diffusing, inactive CP (CP:V-1) feeds, via CARMIL-driven complex exchange, the formation of weak-capping complexes (CP:CARMIL) at the plasma membrane of protruding edges. In vivo, therefore, CARMIL should promote Arp2/3-dependent actin network assembly at the leading edge by promoting barbed-end capping there.
Cell Biology and Physiology Center and.
Ikuko
I
Fujiwara
Cell Biology and Physiology Center and.
Kirsten
K
Remmert
Biophysics Center, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, MD 20892-8017.
Grzegorz
G
Piszczek
Cell Biology and Physiology Center and hammerj@nhlbi.nih.gov.
John A
JA
Hammer
D051344:Actin Capping Proteins / Q000378:metabolism*; D008841:Actin Cytoskeleton / Q000378:metabolism*; D051376:Actin-Related Protein 2-3 Complex / Q000378:metabolism; D000199:Actins / Q000378:metabolism*; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D036341:Intercellular Signaling Peptides and Proteins / Q000378:metabolism*; D007668:Kidney / Q000166:cytology; D051379:Mice; D008840:Microfilament Proteins; D024461:Myosin Type I / Q000378:metabolism; D058105:Polymerization; D011554:Pseudopodia / Q000378:metabolism; D011817:Rabbits; D051381:Rats
D051344:Actin Capping Proteins; D051376:Actin-Related Protein 2-3 Complex; D000199:Actins; C504800:Carmil1 protein, mouse; D002352:Carrier Proteins; D036341:Intercellular Signaling Peptides and Proteins; D008840:Microfilament Proteins; C065700:myotrophin; D024461:Myosin Type I
VASP; cell migration