THE INITIAL ASSOCIATION OF A TRUNCATED FORM OF THE NEUROSPORA PLASMA-MEMBRANE H+-ATPASE AND OF THE PRECURSOR OF YEAST INVERTASE WITH MICROSOMES ARE DISTINCT PROCESSES

Translocation and integration activities were assessed in Neurospora microsomes (nRM) after modification either by a sulfhydryl alkylating reagent or by a proteinase. A Neurospora in vitro system was programmed with RNA transcripts that encode the amino-terminal 194 amino-acid residues of the Neurospora plasma membrane H+-ATPase (pma194+) or the 262 amino-acid residues of the precursor of yeast invertase (preinv262). The processing of preinv262 was blocked in N-phenylmaleimide- and in trypsin-pretreated nRM. In contrast, the binding of preinv262 to microsomes was unaffected in the chemically alkylated nRM, but was affected in the trypsin-pretreated nRM. In the chemically alkylated vesicles, the integration of the pma194+ was not affected, but was partially blocked in the trypsin-pretreated vesicles. These data imply that trypsin-sensitive components are required for these activities in nRM, and that binding, translocation and integration can be differentiated by their sensitivity to chemical alkylation of sulfhydryl groups in nRM. Evaluated also were the effects of temperature on translocation and integration activities in the nRM. These were maximal at 20-degrees-C, whereas the binding of preinV262 was maximal at 0-degrees-C. Taken together, these data demonstrate that the processing of preinv262 by nRM can be resolved into two steps: binding of the precursor protein to nRM and subsequent translocation into the lumen of the vesicles. Whereas, the integration of the pma194+ into nRM could not be resolved into separable steps. Taken together, these results are interpreted to imply that the initial association of truncated forms of the pma+ and the precursor of invertase to the surface of the nRM are distinct processes.