Histidine kinase receptors (HKRs) are part of a two-component system, in which an HKR in the bacterial inner membrane transmits a signal to a response regulator located in the cytoplasm. This is a trans-membrane domain (TM) found in ArcB (class 2, aerobic respiratory control sensor). ArcB has two TM helices connected by a short periplasmic loop. TM domain structures suggests a loose helical packing which provides an inherent flexibility in the TM domains and that this is perhaps essential to the mechanism of signal transduction across the membrane [1].

1: Membrane domain structures of three classes of histidine kinase receptors by cell-free expression and rapid NMR analysis. Maslennikov I, Klammt C, Hwang E, Kefala G, Okamura M, Esquivies L, Mors K, Glaubitz C, Kwiatkowski W, Jeon YH, Choe S; Proc Natl Acad Sci U S A. 2010;107:10902-10907. PMID:20498088