Serine protease that catalyzes the first step (site-1 cleavage) in the proteolytic activation of various factors, prior to site-2 cleavage. Part of a regulated intramembrane proteolysis (RIP) cascade. Cleaves BZIP17 and BZIP28 after the Arg-Arg-Ile-Leu (RRIL) motif. May cleave BZIP49 after the RRIL motif. Targets the membrane-associated BZIP17 factor, which functions as a stress sensor and transducer in a signaling pathway that resembles an ER stress response. Following salt stress, BZIP17 is cleaved by SBT6.1 (S1P) and S2P at the C-terminus and the N-terminal bZIP component is translocated to the nucleus, where it activates the expression of salt stress response genes (PubMed:17662035). Cleaves the pectinesterases PME1 after the Arg-Arg-Leu-Met (RRLM) and Arg-Arg-Leu-Leu (RRLL) motifs, and PME5 after the Arg-Arg-Leu-Leu (RRLL) and Arg-Lys-Leu-Met (RKLM) motifs. This processing and C-terminus release occurs in the Golgi apparatus and is required for cell wall targeting of pectinesterases. Thus, SBT6.1 mediates the regulated release of mature pectinesterases from the Golgi (PubMed:19144003). Cleaves the peptide growth factor RALF23 after the Arg-Arg-Ile-Leu (RRIL) motif. This processing is required for RALF23 function in the negative regulation of brassinolide (BL)-mediated signaling pathway (e.g. BL-induced hypocotyl elongation and branching limitation) (PubMed:19473327). Interacts with PME1 and PME5. Expressed in the vasculature of roots, cotyledons and leaves. Short root (PubMed:20876872). Mutant plants have increased sensitivity to salt-induced osmotic stress (PubMed:17662035). Belongs to the peptidase S8 family.