E3 ubiquitin-protein ligase that mediates ubiquitination and subsequent proteasomal degradation of the transcription factor ICE1. Acts as a negative regulator of cold signaling pathways (PubMed:16702557). Probably involved in recruiting the NUP107-160 subcomplex of the nuclear pore complex to chromatin (Probable). Controls flowering time in response to ambient temperatures (16 and 23 degrees Celsius) and intermittent cold, probably via the regulation of FT and TSF levels (PubMed:22960247). 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. Interacts with SCRM/ICE1, FLK and MSI4/FVE. Nuclear after cold treatment. Cytoplasmic when plant are grown or returned to normal growth temperatures. Ubiquitously expressed with higher levels in leaf vasculature, roots and root tips. Rapid and transient reduction of expression in response to brief cold treatment. After recovery, the level is maintained until 2 days after cold treatment, before declining again. The RING domain is required for E3 ubiquitin ligase activity. The C-terminal part of HOS1 is involved in the interaction with ICE1. Early flowering, and insensitivity to ambient temperature and to intermittent cold, but normal responses to vernalization and gibberellic acid. Loss-of-function mutation (hos1) in the gene shows an early flowering phenotype due to a constitutive vernalization and a reduced freezing tolerance without cold acclimation.