The role of profilin in actin polymerization and nucleotide exchange.
Properties of human profilin I mutated in the major actin-binding site were studied and compared with wild-type profilin using beta/gamma-actin as interaction partner. The mutants ranged in affinity, from those that only weakly affected polymerization of actin to one that bound actin more strongly than wild-type profilin. With profilins, whose sequestering activity was low, the concentration of free actin monomers observed at steady-state of polymerization [Afree], was close to that seen with actin alone ([Acc], critical concentration of polymerization). Profilin mutants binding actin with an intermediate affinity like wild-type profilin caused a lowering of [Afree] as compared to [Acc], indicating that actin monomers and profilin:actin complexes participate in polymer formation. With a mutant profilin, which bound actin more strongly than the wild-type protein, an efficient sequestration of actin was observed, and in this case, the [Afree] at steady state was again close to [Acc], suggesting that the mutant profilin:actin had a greatly lowered ability to incorporate actin subunits at the (+)-end. The results from the kinetic and steady-state experiments presented are consonant with the idea that profilin:actin complexes are directly incorporated at the (+)-end of actively polymerizing actin filaments, while they do not support the view that profilin facilitates polymer formation.
Korenbaum
E
E
Department of Cell Biology, Stockholm University, Sweden.
Nordberg
P
P
Björkegren-Sjögren
C
C
Schutt
C E
CE
Lindberg
U
U
Karlsson
R
R
D000199:Actins / Q000737:chemistry / Q000378:metabolism*; D000818:Animals; D002417:Cattle; D003285:Contractile Proteins; D004985:Ethenoadenosine Triphosphate / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008840:Microfilament Proteins / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D011108:Polymers / Q000378:metabolism*; D051304:Profilins; D011485:Protein Binding; D013329:Structure-Activity Relationship
D000199:Actins; D003285:Contractile Proteins; D004985:Ethenoadenosine Triphosphate; D008840:Microfilament Proteins; C495738:PFN1 protein, human; D011108:Polymers; D051304:Profilins