Is the unique negatively charged polypeptide of crayfish yolk HDL a component of crustacean vitellin?

Abstract

The yolk protein of Cherax quadricarinatus contains six major high‐density lipoprotein (HDL) subunits with the approximate molecular masses of 177, 155, 106, 95, 86, and 75 kDa, of which only the 106‐kDa polypeptide is negatively charged. On the basis of their molecular weights, time of appearance and disappearance, their floating density and susceptibility to enzyme degradation (by a serine proteinase), these six HDL polypeptides were classified into two subgroups. One group comprises the higher‐molecular‐weight compounds above 106 kDa, and the other includes the lower‐molecular‐weight compounds up to 95 kDa. Other than being different from the lower‐molecular‐weight polypeptides, the negatively charged 106‐kDa polypeptide was significantly different from members of its higher‐molecular‐weight group belonging to a different, less abundant, yolk protein as shown by HPLC separation. Immunological studies and peptide mapping in which the 106‐kDa polypeptide did not show similarity to any of the other HDL components confirmed these differences. Moreover, the amino acid composition of the 106‐kDa polypeptide was different from that of known vitellin from other crustacean species. This unique negatively charged polypeptide presents an enigma as it is known to be a secondary vitellogenic‐related HDL polypeptide, immunolocalized in yolk globules; however, it is different to all the other HDL polypeptides, thus presenting the question whether it is indeed a component of “classical” crustacean vitellin. J. Exp. Zool. 290:218–226, 2001.