Title

In vitro characterization of synthetic signal peptide-SecA and -SecY interactions

Date of Completion

January 2004

Keywords

Biology, Cell|Chemistry, Biochemistry

Degree

Ph.D.

Abstract

Correct protein localization is of vital importance for a cell. Transported proteins distinguish themselves from others by the presence of signal peptides at the amino-termini in their precursor sequence. Signal sequences share similar features yet different regions play different roles during the transport process. Idealized model signal sequences have been used to demonstrate important characteristics of signal peptides in the content of preproteins during transport process. SecA and SecY are two major protein components of the bacterial protein transport machinery that are well conserved. SecA functions as a preprotein receptor and a translocation ATPase powering the polypeptide membrane translocation. SecY is the major constituent of the protein conducting channel—translocon. It serves as a membrane receptor for SecA and provides a passageway for the translocating preproteins. ^ This study utilized series of synthetic signal peptides and purified SecA and SecY proteins from Escherichia coli and established specific interactions in vitro between signal peptide, SecA and SecY. ^ Signal peptide-SecA interactions were found to occur similarly in both aqueous solution and in lipid environment and such interactions are primarily hydrophobic-hydrophobic in nature. When the hydrophobic-hydrophobic interaction is less than ideal, ionic interaction can compensate. The extent of SecA-signal peptide interactions in vitro parallels their abilities of supporting preprotein transport in vivo. ^ SecY-signal peptide interactions in detergent micelle are modulated by the presence of SecA and different nucleotides. The signal peptide-binding site on SecY locates primarily at a region including transmembrane segment seven and cytoplasmic loop four of the protein. Transmembrane segment two of SecY could also be involved. ^