Integral membrane proteins

Bacteria other than B. burgdorferi s.l. commonly ensure correct localization of the membrane proteins by including transmembrane-spanning domains in their proteins. Proteins containing such domains appear to be rare in B. burgdorferi s.l.

This is based on the observation that biosynthetic labelling of borrelia cultures with [14C]-amino acids identified only a few amphiphilic polypeptides that do not co-migrate with lipoproteins (Brandt et al., 1990). It was also shown that these putative transmembrane proteins were in relatively low abundance when visualized by freeze-fracture electron microscopy (Walker et al., 1991; Radolf et al., 1994). Virtually none of the proteins were recognized by sera from patients with chronic LB (Brandt et al., 1990). Thus, B. burgdorferi s.l. contains two classes of integral membrane proteins: abundant lipoproteins and rare transmembrane-spanning proteins.

Three characterized membrane proteins have been predicted to contain transmembrane-spanning domains, P66 (Oms66), Oms28 (Oms refers to outer membrane spanning) and P13 (Bunikis et al., 1995; Skare et al., 1996, 1997; Noppa et al., 2001). P66 and Oms28 have also been shown to exhibit porin activities (Skare et al., 1996, 1997). In Gram-negative bacteria, porins are known to form large water-filled channels that allow the diffusion of hydrophilic molecules into the periplasmic space, thus facilitating transport of molecules across the bacterial membrane (Cowan et al., 1992). The third protein, P13, contains trans-membrane-spanning domains and was shown to be surface-exposed by immunofluorescence assays, immunoelectron microscopy and protease sensitivity assays (Sadziene et al., 1995; Noppa et al., 2001). The deduced sequence of the P13 peptide revealed possible signal peptidase type I cleavage sites and the computer analysis predicted that P13 has three transmembrane-spanning domains. Mass spectrometry, in vitro translation and N- and C-terminal amino acid sequencing analyses have indicated that P13 is very unusual as it is post-translationally processed at both ends and modified by an unknown mechanism (Noppa et al, 2001).

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