Studies of the morphology and structure of the plasma lipid transfer particle from the tobacco hornworm, Manduca sexta

Morphological features of Manduca sexta plasma lipid transfer particle (LTP) have been investigated by electron microscopy. LTP was found to be an asymmetric particle with two major structural features: a roughly spherical head and an elongated, hinged tail. The hinge occurs approximately at the midpoint of the tail section with the two halves forming angles ranging from 30 degrees to 180 degrees. A molecular mass estimate of 1.4 x 10(6) daltons based on the dimensions of LTP suggests that multiple copies (two or three) of each of the three LTP apoproteins exist in the native complex. Limited digestion studies of LTP suggest that apoLTP-III is less susceptible to trypsin cleavage than apoLTP-I or -II, and therefore may be less exposed to the aqueous environment. Digestion for 1 h at a 1:50 trypsin-LTP protein ratio did not alter the flotation properties of LTP or its morphological features; thus, although significant proteolysis occurred, the particle retained its overall structure. Transfer activity, on the other hand, was affected by trypsin digestion with 30 +/- 14% inhibition of LTP activity occurring upon proteolysis at a 1:50 trypsin-LTP protein ratio. Treatment of LTP with phospholipase A2 resulted in the conversion of LTP-associated phosphatidylcholine and phosphatidylethanolamine to their corresponding lyso forms. Phospholipase A2 treatment did not, however, alter the SDS-PAGE profile, transfer activity, flotation pattern, or the microscopic features of LTP. These results suggest that the products of the phospholipase reaction remain associated with the particle.(ABSTRACT TRUNCATED AT 250 WORDS)