TABLE 5.

Putative membrane-based osmosensors^a

Osmoregulatory role	Function	Name	Organism	Purified/osmosensing shown in vitro
Unclear	Sensor kinase	EnvZ	E. coli	Yes/No
K+ accumulation	Transporter	TrkEFG(H)	E. coli	No/No
	Sensor kinase	KdpD	E. colib	Yes/No
	P-type ATPase	KdpFABC	E. colib	Yes/No
Compatible-solute accumulation	Ion-linked transporter	ProP	E. coli	Yes/Yes
			S. typhimurium	No/No
		BetT	E. coli	No/No
		BetP	C. glutamicum	No/No
		OpuE	B. subtilis	No/No
	ABC transporter	ProU	E. coli	No/No
		OpuA	B. subtilis	No/No
		OpuC	B. subtilis	No/No
Cosolvent efflux	Channel	MscL	E. coli	Yes/Yes

^aThe listed proteins have been shown to function as osmosensors (EnvZ, KdpD, and ProP) and/or as osmoregulators (all but EnvZ and KdpD) in vivo and/or in vitro, and their amino acid sequences are known. Related systems in other organisms have also been characterized (see text). 

^bSystems structurally and functionally related to E. coli Kdp have been identified in a variety of other organisms (references 278 and 289 and references therein).