A second isoform of 3-ketoacyl-CoA thiolase found in Caenorhabditis elegans, which is similar to sterol carrier protein x but lacks the sequence of sterol carrier protein 2.
We cloned a full-length cDNA of the nematode Caenorhabditis elegans that encodes a 44-kDa protein (P-44, 412 residues) similar to sterol carrier protein x (SCPx). Mammalian SCPx is a bipartite protein: its 404-residue N-terminal and 143-residue C-terminal domains are similar to 3-ketoacyl-CoA thiolase and identical to the precursor of sterol carrier protein 2 (SCP2; also termed non-specific lipid-transfer protein), respectively. P-44 has 56% sequence identity to the thiolase domain of SCPx but lacks the SCP2 sequence. Northern blot analysis revealed only a single mRNA species of 1.4 kb, which agrees well with the length of the cDNA (1371 bp), making it improbable that alternative splicing produces an SCPx-like fusion protein. The sequence similarities of P-44 to conventional thiolases are lesser than that to SCPx. Purified recombinant P-44 cleaved long-chain 3-ketoacyl-CoAs (C(8-16)) in a thiolytic manner by the ping-pong bi-bi reaction mechanism. The inhibition of P-44 by acetyl-CoA was competitive with CoA and non-competitive with 3-ketooctanoyl-CoA. This pattern of inhibition is shared with SCPx but not with conventional 3-ketoacyl-CoA thiolase, which is inhibited uncompetitively with respect to 3-ketoacyl-CoA. From these results, we concluded that nematode P-44 and mammalian SCPx constitute a second isoform of thiolase, which we propose to term type-II 3-ketoacyl-CoA thiolase.
Bun-Ya
M
M
Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima, Japan.
Maebuchi
M
M
Hashimoto
T
T
Yokota
S
S
Kamiryo
T
T
D000101:Acetyl-CoA C-Acetyltransferase / Q000737:chemistry* / Q000235:genetics; D000102:Acetyl-CoA C-Acyltransferase / Q000737:chemistry*; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D017173:Caenorhabditis elegans / Q000201:enzymology*; D002352:Carrier Proteins / Q000737:chemistry* / Q000235:genetics; D002417:Cattle; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000737:chemistry; D006801:Humans; D007527:Isoenzymes / Q000737:chemistry*; D007700:Kinetics; D008969:Molecular Sequence Data; D010940:Plant Proteins; D013261:Sterols / Q000737:chemistry*
D002352:Carrier Proteins; D018076:DNA, Complementary; D007527:Isoenzymes; D010940:Plant Proteins; D013261:Sterols; C072087:sterol carrier proteins; D000102:Acetyl-CoA C-Acyltransferase; D000101:Acetyl-CoA C-Acetyltransferase