Possesses E3 ubiqutin-protein ligase activity in vitro. Acts as negative regulator of salinity stress tolerance mediated by the ubiquitin-proteasome degradation pathway. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Induced by salt stress in roots (PubMed:27118216). Induced by heat shock, drought stress and abscisic acid (ABA) (PubMed:27118216). The RING-type zinc finger domain mediates binding to an E2 ubiquitin-conjugating enzyme. Plants over-expressing SIRP1 exhibit enhanced sensitivity to seed germination and root growth inhibition by salt stress.