New insights into S2P signaling cascades: regulation, variation, and conservation

Abstract

Regulated intramembrane proteolysis (RIP) is a conserved mechanism that regulates signal transduction across the membrane by recruiting membrane-bound proteases to cleave membrane-spanning regulatory proteins. As the first identified protease that performs RIP, the metalloprotease site-2 protease (S2P) has received extensive study during the past decade, and an increasing number of S2P-like proteases have been identified and studied in different organisms; however, some of their substrates and the related S1Ps remain elusive. Here, we review recent research on S2P cascades, including human S2P, E. coli RseP, B. subtilis SpoIVFB and the newly identified S2P homologs. We also discuss the variation and conservation of characterized S2P cascades. The conserved catalytic motif of S2P and prevalence of amino acids of low helical propensity in the transmembrane segments of the substrates suggest a conserved catalytic conformation and mechanism within the S2P family. The review also sheds light on future research on S2P cascades.

Figure 1

Schematic drawings showing the S2P cascades of human S2P, E. coli RseP and B. subtilis SpoIVFB. Substrates are shown in orange, Site-2 proteases are shown in yellow. The white scissors represent the cleavage by Site-2 proteases while the black scissors represent the cleavage by Site-1 proteases or other proteases. The metal chelating motif that includes HExxH and D is shown in black. A: Under low levels of sterols, human SREBP is transported from the ER membrane to the Golgi to be sequentially cleaved by S1P and S2P. B: In E. coli, RseA is cleaved sequentially by DegS and RseP. C: After activation by SpoIVB, SpoIVFB cleaves pro-σ^k to release mature σ^k.

Figure 2

Sequence alignment and relationship among S2P homologs. The sequences of 18 characterized S2P homologs are presented. Only the core TM domains containing the HEXXH and NPDG motifs are shown. A: Sequence alignment in the vicinity of the HEXXH motif. The presumed transmembrane helix (TM) is boxed and the consensus amimo acids are highlighted in gray. B: Sequence alignment for the NPDG motif. C: Unrooted dendrogram of S2P homologs based on the alignment of the HEXXH and NPDG motifs. TMpred (http://www.ch.embnet.org/software/TMPRED_form.html) and PSIPRED Protein Structure Prediction Server (http://bioinf.cs.ucl.ac.uk/psipred/) were used to predict TM helix. The unrooted dendrogram was generated by CLUSTALW (http://align.genome.jp/). The aligned sequences are: H.sapiens S2P (GI:6016601), D. melanogaster S2P (GI:19922044), C. elegans S2P (GI:22859116), C. neoformans Stp1 (BROAD ID: CNAG_05742), A. thaliana EGY1 (GI:15238440), Synechocystis sp. PCC6803 sll0862 (GI:16330216), Synechocystis sp. PCC6803 slr0643 (GI:16331565), A. thaliana AraSP (GI:18402981), E.coli RseP (GI:16128169), V. cholerae Vc-RseP (GI:20978850), P. aeruginosa MucP (GI:146448760), B. bronchiseptica HurP (GI:33601589), C. crescentus MmpA (GI:20978837), B. subtilis RasP (GI:20978800), E. faecalis Eep (GI: 256853722), M. tuberculosis Rv2869c (GI:20978863), B. subtilis SpoIVFB (GI:16079849), M. jannaschii mjS2P (GI:2499926).

Figure 3

Sequence and properties of S2P substrates. The sequences of the predicted TM segments of 14 identified S2P substrates are shown. The boxes denote the predicted TM domains. Putative helix-destabilizing residues (N, P, G or repeated amino acids) in the TM regions are highlighted in gray. S1P and S2P cleavage sites, where known, are highlighted between residues in bold (outside TM for S1P and inside or close to TM for S2P, respectively). The consensus sequences of the human S1P site (RxxL or RxLx) are underlined. The aligned sequences are: H. sapiens SREBP2 (GI:27477113), H. sapiens ATF6 (GI:2245630), H. sapiens CREBH (GI:14211949), H. sapiens OASIS (GI:21668502), C. neoformans Sre1 (BROAD ID: CNAG_04804), E. coli RseA (GI:89109378), P. aeruginosa MucA (GI:223702393), V. cholerae TcpP (GI:14548354), B. bronchiseptica HurR (GI:33603626), C. crescentus PodJ (GI:221235062), B. subtilis RsiW (GI:62900894), B. subtilis FtsL (GI:1122761), M. tuberculosis pbpB (GI:15609300), B. subtilis pro-σ^k (GI:133478).

Figure 4

The crystal structure of mjS2P from Methanocaldococcus jannaschii. Six transmembrane helices (TM) are shown in different colors. The catalytic zinc atom (red) is coordinated by His54 and His58 on TM2 and Asp148 at the N terminal end of helix TM4C. The conserved Glu55 and Asn140 may contribute to catalysis.