ACYL-COA-BINDING AND TRANSPORT, AN ALTERNATIVE FUNCTION FOR DIAZEPAM BINDING INHIBITOR (DBI), WHICH IS IDENTICAL WITH ACYL-COA-BINDING PROTEIN

Acyl-CoA binding protein (ACBP) was originally identified as an artifact in a preparation of fatty acid binding protein. The amino acid sequence of ACBP from bovine, rat and human liver is identical to the sequence of diazepam binding inhibitor (DBI) from these species. ACBP and DBI are therefore one and the same protein. The tertiary structure of ACBP in solution has been determined by 2D-NMR. ACBP consists of 4 alpha-helixes, covering the sequence from amino acid 2-11, 20-38, 51-62 and 72-85, respectively. The protein is folded so that it forms a boomerang type of structure with helix 1 and 2 arranged antiparallel in the one arm of the boomerang, helix 3 and the non-helical part between helix 2 and 3 form the second arm in the boomerang. Helix 4 is located in an angle behind helix 1 and 2. NMR measurements of chemical shifts, induced by acyl-CoA binding, indicate that the binding site is located in the bottom of the V formed between the two arms of the boomerang. This location of the binding site is confirmed with affinity labelling with radioactive photoreactive acyl-CoA esters. ACBP does not bind free CoA or free fatty and short chain acyl-CoA esters (C2-C8). The affinity increases with increasing length of the acyl chain from C10-C20 and drops again in acyl-CoA esters with 22 and 24 carbon in the acyl chain. Increasing the number of double bonds in the acyl chain only has a small effect on binding affinity of ACBP for acyl-CoA esters. ACBP effectively absorbs acyl-CoA esters from microsomal membranes and multilaminar vesicles and protects them against hydrolysis by microsomal thioesterases. The ACBP acyl-CoA complex can denote acyl-CoA esters to acyl-CoA cholesterol acyltransferase. Evidence is provided that ACBP may play an important role in connection with regulation of acetyl-CoA carboxylase, which is inhibited by acyl-CoA esters.