Arabidopsis capping protein (AtCP) is a heterodimer that regulates assembly at the barbed ends of actin filaments.
The precise regulation of actin filament polymerization and depolymerization is essential for many cellular processes and is choreographed by a multitude of actin-binding proteins (ABPs). In higher plants the number of well characterized ABPs is quite limited, and some evidence points to significant differences in the biochemical properties of apparently conserved proteins. Here we provide the first evidence for the existence and biochemical properties of a heterodimeric capping protein from Arabidopsis thaliana (AtCP). The purified recombinant protein binds to actin filament barbed ends with Kd values of 12-24 nM, as assayed both kinetically and at steady state. AtCP prevents the addition of profilin actin to barbed ends during a seeded elongation reaction and suppresses dilution-mediated depolymerization. It does not, however, sever actin filaments and does not have a preference for the source of actin. During assembly from Mg-ATP-actin monomers, AtCP eliminates the initial lag period for actin polymerization and increases the maximum rate of polymerization. Indeed, the efficiency of actin nucleation of 0.042 pointed ends created per AtCP polypeptide compares favorably with mouse CapZ, which has a maximal nucleation of 0.17 pointed ends per CapZ polypeptide. AtCP activity is not affected by calcium but is sensitive to phosphatidylinositol 4,5-bisphosphate. We propose that AtCP is a major regulator of actin dynamics in plant cells that, together with abundant profilin, is responsible for maintaining a large pool of actin subunits and a surprisingly small population of F-actin.
Huang
Shanjin
S
Department of Biological Sciences and Purdue Motility Group, Purdue University, West Lafayette, Indiana 47907-2064, USA.
Blanchoin
Laurent
L
Kovar
David R
DR
Staiger
Christopher J
CJ
D051339:Actin Depolymerizing Factors; D000199:Actins / Q000737:chemistry / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D017360:Arabidopsis / Q000235:genetics; D029681:Arabidopsis Proteins / Q000737:chemistry* / Q000235:genetics / Q000502:physiology*; D051347:CapZ Actin Capping Protein; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D018076:DNA, Complementary / Q000737:chemistry; D051345:Destrin; D015870:Gene Expression; D007526:Isoelectric Point; D008840:Microfilament Proteins / Q000737:chemistry* / Q000235:genetics / Q000502:physiology*; D008856:Microscopy, Fluorescence; D008969:Molecular Sequence Data; D008970:Molecular Weight; D016296:Mutagenesis; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000494:pharmacology; D011108:Polymers / Q000737:chemistry; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics; D011994:Recombinant Proteins; D016415:Sequence Alignment; D013329:Structure-Activity Relationship
D051339:Actin Depolymerizing Factors; D000199:Actins; D029681:Arabidopsis Proteins; D051347:CapZ Actin Capping Protein; C495843:Capza1 protein, mouse; D018076:DNA, Complementary; D051345:Destrin; D008840:Microfilament Proteins; D019269:Phosphatidylinositol 4,5-Diphosphate; D011108:Polymers; D021122:Protein Subunits; D011994:Recombinant Proteins