SLN1 (YIL147C, UniProt P39928) curation notes

Identity and architecture

• 1220 aa hybrid sensor histidine kinase; two predicted TM helices (23-46, 334-354) flanking an
  extracellular (periplasmic) domain (47-333); cytoplasmic HAMP, HisKA/HATPase transmitter
  (573-928, phospho-His576) and C-terminal CheY-like receiver (1089-1210, phospho-Asp1144)
  [file:yeast/SLN1/SLN1-uniprot.txt "Histidine kinase that acts as an osmosensor at the plasma membrane."].
• PANTHER PTHR43047:SF72 (two-component histidine protein kinase family); no HPt domain.

Biochemistry: phosphorelay

• Posas et al. 1996 (abstract only in cache): PMID:8808622 and
  PMID:8808622
• Ault et al. 2002 (full text): Sln1 receiver is an obligate intermediate between kinase and Ypd1
  PMID:12455952;
  activated allele sln1-22 (P1148S) shifts the phosphotransfer equilibrium toward Ypd1
  PMID:12455952.
• Ostrander & Gorman 1999 (full text): ECD acts as a dimerization/activation domain; TMD1 required for
  osmotic regulation PMID:10198019.
• Structure of Ypd1 in complex with the Sln1 receiver domain (1OXB/1OXK/2R25)
  PMID:14656441.

Signalling logic

• Active (phosphorylated) under low osmolarity; hyperosmotic stress / loss of turgor inactivates
  Sln1, Ssk1 becomes unphosphorylated and activates Ssk2/Ssk22 -> Pbs2 -> Hog1
  PMID:12821642.
• Second branch: Ypd1 -> Skn7 Asp427 (nuclear TF; OCH1, TRX2 targets)
  PMID:9843501.
• sln1 null lethality is due to HOG hyperactivation (suppressed by ssk1, ssk2, pbs2, hog1 deletion)
  [file:yeast/SLN1/SLN1-deep-research-falcon.md, section 4.1].

Sensing

• Turgor sensor: nystatin, cell-wall removal and hyperosmotic stress all inactivate Sln1; periplasmic
  aa 138-150 essential PMID:12821642. Exact physical variable unresolved.

Localization

• Plasma membrane: fractionation PMID:10198019;
  Sln1-GFP at cell rim PMID:14665464.

Annotation issues noted

• GO:0009927 histidine phosphotransfer kinase activity: current definition ("Serves as a phospho-His
  intermediate enabling the transfer of phospho group between a hybrid kinase and a response
  regulator") describes the HPt role, which in yeast is Ypd1. Sln1 is the hybrid kinase donor and
  lacks an HPt domain. However, SGD (IDA+IMP, PMID:12455952), EcoCyc/EcoliWiki (BarA), TAIR (AHK1)
  and PAINT (PTHR43047 node) all apply it to hybrid kinases too, so this is a community convention
  (possibly predating the definition revision). Kept as non-core; raised as a question.
• GO:0005515 (Ypd1, PMID:14656441): uninformative; the relevant function (phosphodonor to Ypd1) is
  captured by GO:0000155 / phosphorelay. REMOVE (interaction itself is real).
• No NEW terms proposed: GO:0007234 already captures the process; the Sln1-Skn7 branch (OCH1
  transcription) is downstream output via Ypd1/Skn7 and would be indirect.