Self-association of transmembrane domain 2 (TM2), but not TM1, in carnitine palmitoyltransferase 1A: role of GXXXG(A) motifs.

Carnitine palmitoyltransferase 1 (CPT1) controls the rate of entry of long-chain fatty acids into the mitochondrial matrix for beta-oxidation and has been reported to exist as an oligomer. We have investigated the in vivo oligomerization of full-length rat CPT1A (rCPT1A) along with those of the N-terminal truncation/deletion mutants Delta(1-82), Delta(1-18), and Delta(19-30) expressed in yeast mitochondria. The data indicate that in liver mitochondria in vivo CPT1A exists as a hexamer but that during preparation and storage of mitochondria the order of oligomerization is rapidly reduced to the trimer, such that a mixture of hexamer and trimer is observed in isolated mitochondria in vitro. Mutants bearing deletions of different segments of the N terminus (including the more N-terminal of the two transmembrane domains) have the same pattern of oligomerization when expressed in yeast mitochondria. The self-association of the individual rCPT1A transmembrane (TM) domains (TM1, TM2) was also studied using the TOXCAT assay (which measures TM self-association in the Escherichia coli inner membrane). There was minimal self-association of the sequence corresponding to TM1 but significant self-association of TM2 in TOXCAT. Chemical cross-linking and analytical ultracentrifugation of a TM2-derived synthetic peptide showed oligomerization with a similar trimer/hexamer equilibrium to that observed for native rCPT1A in isolated mitochondria. Therefore, there was a correlation between the oligomerization behavior of TM2 peptide and that of the full-length protein. In silico molecular modeling of rCPT1A TM2 highlighted the favorable orientation of GXXXG and GXXXA motifs in the formation of the TM2 hexamer.