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Myristate exchange in glycolipid A and VSG of African trypanosomes.

The variant surface glycoprotein (VSG) of T. brucei is anchored to the plasma membrane via a glycosylphosphatidylinositol (GPI) anchor which is unique in that its fatty acids are exclusively myristate (a fourteen carbon saturated fatty acid). We showed that the myristate is added to the GPI precursor in a remodeling reaction involving deacylation and reacylation. We now demonstrate that trypanosomes have a second pathway of myristoylation for GPI anchors that we call "myristate exchange" which is distinct from the fatty acid remodeling pathway. We propose that this is an exchange of [3H]myristate into both sn-1 and sn-2 positions of glycolipid A, which already contains myristate, and have demonstrated this using inhibitors and a variety of other methods. We have partially characterized myristate exchange with respect to specificity and susceptibility to some inhibitors. The apparent Km for myristoyl CoA is 7 nM. This myristate-specific process may represent a proof-reading system to ensure that the fatty acids on VSG are exclusively myristate. Although myristate exchange was first discovered for glycolipid A, we now believe that VSG is the true substrate of this reaction. VSG is efficiently labeled by exchange in the presence of cycloheximide, which prevents anchoring of newly synthesized protein. Although its location is not yet known, we have evidence that exchange does not localize to either the endoplasmic reticulum or the plasma membrane. We will present data indicating that surface VSG may be internalized and undergo myristate exchange.