PROGRESS TOWARDS THE CRYSTAL STRUCTURE OF FFH, THE BACTERIAL HOMOLOG OF SRP54. Douglas Freymann, Robert Keenan, Robert Stroud & Peter Walter, Dept. of Biochemistry, University of California, San Francisco, CA 94143-0448.

Ffh is the bacterial homolog of the signal sequence recognition protein SRP54 - we have obtained diffraction quality crystals of both the intact Ffh and its amino-terminal domain. The signal recognition particle, SRP, in eukaryotes comprises six proteins and one RNA, and functions to target nascent secretory and membrane proteins to the translocation apparatus of the endoplasmic reticulum. In prokaryotes a ribonucleoprotein with similar function comprises at least one protein, Ffh, and the 4.5S RNA. SRP54 and Ffh have been shown to mediate recognition of the signal sequence of the nascent polypeptide. They are multi-domain proteins. The amino-terminal `NG'-domain is a GTPase whose activity is modulated by interaction with the C-terminal methionine-rich `M'-domain (which interacts directly with the signal peptide and with the RNA component of the particle) and by interaction with the membrane associated SRP receptor. We seek to understand the structural basis of signal sequence recognition and the function of GTP binding and hydrolysis in targeting. Progress towards the structure determination of Ffh will be reported.