Identification of diverse archaeal proteins with class III signal peptides cleaved by distinct archaeal prepilin peptidases

From the Department of Molecular Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands; Center for Tropical & Emerging Global Diseases & Department of Genetics, University of Georgia, C210 Life Sciences, Athens, GA 30602-7223; Biology Department, University of Pennsylvania, 415 University Avenue, Philadelphia, PA 19104

^* To whom correspondence should be addressed. Email: pohlschr{at}sas.upenn.edu.

	Abstract

Most secreted archaeal proteins are targeted to the membrane via a tripartite signal composed of a charged N-terminus and a hydrophobic domain followed by a signal peptidase processing site. Signal peptides of archaeal flagellins, similar to class III signal peptides of bacterial type IV pilins, are distinct in that their processing site precedes the hydrophobic domain, which is crucial for assembly of these extracytoplasmic structures. To identify the complement of archaeal proteins with class III signal sequences, a PERL program (FlaFind) was written. A diverse set of proteins was identified and many of these FlaFind positives were encoded by genes that were co-transcribed with homologs of pilus assembly genes. Moreover, structural conservation of primary sequences between many FlaFind positives and subunits of bacterial pilus-like structures, which have been shown to be critical for pilin assembly, have been observed. A subset of pilin-like FlaFind-positives contained a conserved domain of unknown function (DUF361) within the signal peptide. Many of the genes encoding these proteins were in operons that contained a gene encoding a novel euryarchaeal prepilin-peptidase, EppA homolog. Heterologous analysis revealed that Methanococcus maripaludis Duf361-containing proteins were specifically processed by the EppA homolog of this archaeon. Conversely, M. maripaludis preflagellins were only cleaved by the archaeal prepilin peptidase, FlaK. Together, the results reveal a diverse set of archaeal proteins with class III signal peptides that might be subunits of as yet, undescribed, cell-surface structures, such as archaeal pili.