Correlation between polymerizability and conformation in scallop beta-like actin and rabbit skeletal muscle alpha-actin.
In order to investigate the structural basis for functional differences among actin isoforms, we have compared the polymerization properties and conformations of scallop adductor muscle beta-like actin and rabbit skeletal muscle alpha-actin. Polymerization of scallop Ca(2+)-actin was slower than that of skeletal muscle Ca(2+)-actin. Cleavage of the actin polypeptide chain between Gly-42 and Val-43 with Escherichia coli protease ECP 32 impaired the polymerization of scallop Mg(2+)-actin to a greater extent than skeletal muscle Mg(2+)-actin. When monomeric scallop and skeletal muscle Ca(2+)-actins were subjected to limited proteolysis with trypsin, subtilisin, or ECP 32, no differences in the conformation of actin subdomain 2 were detected. At the same time, local differences in the conformations of scallop and skeletal muscle actin subdomains 1 were revealed as intrinsic fluorescence differences. Replacement of tightly bound Ca(2+) with Mg(2+) resulted in more extensive proteolysis of segment 61-69 of scallop actin than in the case of skeletal muscle actin. Furthermore, segment 61-69 was more accessible to proteolysis with subtilisin in polymerized scallop Ca(2+)-actin than in polymerized skeletal muscle Ca(2+)-actin, indicating that, in the polymeric form, the nucleotide-containing cleft is in a more open conformation in beta-like scallop actin than in skeletal muscle alpha-actin. We suggest that this difference between scallop and skeletal muscle actins is due to a less efficient shift of scallop actin subdomain 2 to the position it has in the polymer. The possible consequences of amino acid substitutions in actin subdomain 1 in the allosteric regulation of the actin cleft, and hence in the different stabilities of polymers formed by different actins, are discussed.
Khaitlina
S
S
Institute of Cytology RAS, Tykhoretsky av. 4, St. Petersburg, 194064, Russia.
Antropova
O
O
Kuznetsova
I
I
Turoverov
K
K
Collins
J H
JH
D000199:Actins / Q000737:chemistry*; D000495:Allosteric Site; D019943:Amino Acid Substitution; D000818:Animals; D001704:Biopolymers / Q000737:chemistry; D002118:Calcium / Q000737:chemistry; D008274:Magnesium / Q000737:chemistry; D008974:Mollusca; D018482:Muscle, Skeletal / Q000737:chemistry*; D011487:Protein Conformation; D011817:Rabbits
D000199:Actins; D001704:Biopolymers; D008274:Magnesium; D002118:Calcium