GIARDIA-LAMBLIA - TRAFFIC OF A TROPHOZOITE VARIANT SURFACE PROTEIN AND A MAJOR CYST WALL EPITOPE DURING GROWTH, ENCYSTATION, AND ANTIGENIC SWITCHING

Both trophozoites and cysts of Giardia lamblia have-unique outer surfa ces that protect them from very different hostile environments. Howeve r, little is known about the transport of these important molecules to the cell surface. We used monospecific anti-recombinant TSA 417 antib odies and mAb 8C5 in double label immunoelectron microscopy to compare the localization and transport of this major trophozoite surface anti gen (TSA) with that of a prominent cyst wall epitope during vegetative growth, encystation, and antigenic switching in vitro. TSA 417 is a m arker of the constitutive transport of the major plasma membrane prote in, while the 8C5 epitope traces a differentiation-regulated secretory pathway to the cyst wall. Both proteins localized to the nuclear enve lope endoplasmic reticulum (ER) cisternae, ER, and cytoplasmic membran e cisternae, reflecting their site of synthesis, but only the differen tiation-specific epitope 8C5 localized to the encystation-specific ves icles (ESV). These large secretory vesicles form only during encystati on and transport cyst antigens to the nascent wall. In contrast, only TSA 417 was found on the outer surface of the plasmalemma of trophozoi tes and encysting cells and underlaying the walls of many cysts, while only 8C5 localized to the cyst wall. As encystation progressed, TSA 4 17 disappeared from the plasmalemma and increased in the lysosome-like PV and other large cytoplasmic vesicles. In contrast to their segrega tion in the ESV and on the cell surface, both TSA 417 and 8C5 were fou nd in the peripheral vesicles, presumably an endocytic compartment. We propose that this may be the initiation of a stage in differentiation -driven antigenic switching of TSA 417, in which this antigen is no lo nger synthesized or exported to the plasmalemma, but is taken back ins ide the cell. (C) 1994 Academic Press, Inc.