Catalyzed insertion of proteins into phospholipid membranes: specificity of the process

The process of insertion of intrinsic proteins into phospholipid membranes conjures up the thought of enormous energy barriers but is a routine occurrence in cells. Proteinaceous complexes responsible for protein targeting/translocation/insertion into membranes have been studied intensively. However, the mitochondrial voltage-dependent anion channel (VDAC), can insert into phospholipid membranes by an auto-catalytic process called "auto-directed insertion." This process results in an oriented insertion of VDAC channels and an increase in insertion rate per unit area of 10 orders of magnitude. Here we report that VDAC catalyzes the insertion of PorA/C1 and KcsA by increasing their calculated insertion rate per unit area by 9 orders of magnitude with no detectable effect on the insertion of alpha-hemolysin. This was measured as a reduction in the delay before the first insertion of these proteins. Gramicidin and PorA/C1 accelerate the calculated insertion rate per unit area of VDAC by 8 and 9 orders of magnitude, respectively. Only PorA/C1 increases the overall rate of VDAC insertion (50-fold) over the self-catalyzed rate. Our results indicate that catalyzed insertion of proteins into phospholipid membranes does not arise simply from disturbance of the phospholipid membrane because it shows strong specificity.