Investigation and characterisation of transmembrane domain interactions in Carnitine Palmitoyltransferase 1 Isoforms

Carnitine Palmitoyltransferase 1 (CPT1) is an outer mitochondrial membrane protein that plays an important role in the ß-oxidation of long chain fatty acids by regulating their entry into the mitochondrial matrix. This regulatory property is due to its inhibition by malonyl-CoA. Three isoforms of CPT1 have been identified: CPT1A (which is abundant in the liver); CPT1B (which occurs in heart and skeletal muscles and other highly oxidative tissues e.g. brown adipose) and a brain isoform (CPT1C). CPT1A and CPT1B have a high degree of similarity in primary sequence, however CPT1B has 30-100 fold greater sensitivity to malonyl-CoA. CPT1A and CPT1B are both predicted to have two transmembrane (TM) domains which are thought to interact both inter- and intramolecularly, and to sense the membrane environment in which they occur. These interactions are thought to be important for the structure and the function of CPT1. The potential for modulating the function of the three different CPT1 isoforms is of therapeutic interest for the treatment of many metabolic disorders, most notably diabetes. The main focus of this research was to systematically study the TM domains of CPT1A and CPT1B to characterise the homotypic and heterotypic interactions that occur between them. Using an in vivo assay designed to measure interactions between TM domains in the E. coli inner membrane, it has been shown that the TM domains in CPT1A and CPT1B interact, in both a homotypic and a heterotypic fashion, to a relatively strong degree. Using mutagenesis, several important residues have been identified in the stabilisation of homotypic and heterotypic interactions in the TM domains of CPT1A and CPT1B in this manner. The residues identified illustrate some key differences and similarities between the interaction profiles of these two isoforms. The CPT1B TM domains were also expressed and purified as peptides and in vitro biophysical experiments were used to corroborate interaction data in the full TM domains of CPT1B