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This paper describes the detailed structure of a membrane-embedded aspartyl protease called Flak, a preflagellin peptidase (PFP) from an extremophile that has been categorized as a "GXGD aspartyl protease," along with presenilin and SPP. As this is the first structure of a membrane-embedded aspartyl protease, it is quite important, and Ha and his coauthors deserve considerable credit, as working with such proteases and elucidating their structure is highly challenging. The FlaK structure should be useful for testing specific hypotheses about how this and (truly) related enzymes work, just as the structure of the E. coli
Rhomboid GlpG has allowed testing of specific ideas for how transmembrane substrates gain access to the interior Ser-His active site dyad.
The structure shows two aspartates close, but not close enough to be catalytically active. The conditions used for crystallization (e.g., pH 9.5) are apparently capturing an inactive conformation, and so it is unclear exactly what the active conformation looks like. One can only guess about the changes needed to...