Integral membrane histidine sensor kinase, DesK or YocF, with 370 aas and 5 or 6 N-terminal TMSs plus a C-terminal kinase domain. It aligns with residues 20 - 420 of the histidine kinase with TC# 9.B.238.3.1. It is a member of the two-component regulatory system DesR/DesK (Albanesi et al. 2004), responsible for cold induction of the des gene coding for the Delta5 acyl-lipid desaturase (Aguilar et al. 2001). It acts as a sensor of membrane fluidity (Cybulski et al. 2002) and probably activates DesR by phosphorylation (Hunger et al. 2004). The transmembrane region triggers phosphorylation of the catalytic domain as soon as the membrane lipids rigidify. Interhelical H-bonds modulate its activity (Almada et al. 2021).