Fatty acid remodeling of glycosyl phosphatidylinositol anchors in Trypanosoma brucei: incorporation of fatty acids other than myristate

Trypanosoma brucei is the protozoan parasite that causes African sleeping s ickness. Its surface is packed with 10(7) copies of the glycosyl phosphatid ylinositol (GPI)-anchored variant surface glycoprotein (VSG). This GPI anch or is unusual in that it contains two myristates (14:0) in its lipid moiety . This fatty acid specificity is achieved through myristoylation of the GPI precursor, and the acyltransferases involved in the GPI remodeling were pr esumed to be specific for myristate. However, their specificity had never b een fully evaluated. Here we found as expected that the remodeling acyltran sferases completely excluded palmitate (16:0) and stearate (18:0) in a cell -free fatty acid remodeling system. In contrast, we found surprisingly that one of these enzymes was permissive to shorter Fatty acids such as laurate (12:0) and octanoate (8:0). However, the rates of incorporation of shorter fatty acids were lower than that of myristate at low substrate concentrati on. Since shorter fatty acids are virtually absent in the parasite and in t he host bloodstream, it is unlikely that shorter fatty acids compete effect ively with myristate as remodeling substrates under physiological condition s. Even if they were present in small quantities, a recently identified spe cialized Fatty acid synthetase efficiently elongates shorter fatty acids to myristate prior to incorporation into GPIs (Morita et al., Science 288 (20 00) 140-3.). Therefore, even though a remodeling acyltransferase is permiss ive with regard to substrate chain length, the myristate specificity in CPI anchors is very high. (C) 2001 Elsevier Science B.V. All rights reserved.