Analysis of haloarchaeal twin-arginine translocase pathway reveals the diversity of the machineries

Abstract

The twin-arginine translocase (Tat) pathway transports folded proteins across the plasma membrane and plays a critical role in protein transport in haloarchaea. Computational analysis and previous experimental evidence suggested that the Tat pathway transports almost the entire secretome in haloarchaea. The TatC, receptor component of this pathway shows greater variation in membrane topology in haloarchaea than in other organisms. The presence of a unique fourteen-transmembrane TatC homolog (TatC_t) in haloarchaea, over and above the expected TatC topological variants, indicates a strong correlation between the additional homologs and the large number of substrates transported via the haloarchaeal Tat pathway. Various combinations of TatC homologs with different topologies-TatC_o, TatC_t, TatC_n, and TatC_x have been observed in haloarchaea. In this report, on the basis of these combinations we have segregated all haloarchaeal Tat substrates into two groups. The first group consists of substrates that are transported by TatC_t alone, whereas the second group consists of substrates that are transported by the other TatC homologs (TatC_o, TatC_n, and TatC_x). The various haloarchaea TatA components also shows the possible segregation towards the substrates. We have also identified the possible homologs for Tat substrate chaperones, which act as a quality-control mechanism for proper protein folding. Further sequence analysis implies that the two TatC domains of TatC_t complement each other's functionally. Substrate analysis also revealed subtle differences between the substrates being transported by various homologs: further experimental analysis is therefore required for better understanding of the complexities of the haloarchaeal Tat pathway.

Keywords: Microbiology.

Fig. 1

Schematic representation of different TatC topology and grouped in to different groups based on combinations. TatC_t + TatC_o, TatC_t + TatC_n and TatC_t + TatC_x which are denoted in group 1, Group 2 and group 3, respectively.

Fig. 2

The alignment between N and C terminal TatC domain of TatC_t.

Fig. 3

Phylogenetic analysis of all TatC_t from haloarchaea. Group 3 TatC_t shown in bracket and group 1 members are highlighted; the remaining members from group 2 group.

Fig. 4

The whisker plot representation of GC content of fifty genes flanking either side of the TatC (represented as filled dot) of respective organism.

Fig. 5

Phylogenetic analysis of TatA/E from 20 haloarcheal species studies. Group 1 members are highlighted in yellow, group 2 in green and group 3 in blue.

Fig. 6

a) Weblogo representation of Tat signal motifs in each class of substrates. b) Weblogo representation of Tat signal peptide regions from different classes of substrates showing the occurrence of lipobox motif (AGC) in the shared substrates. WebLogo was generated at http://weblogo.berkeley.edu/logo.cgi.

Fig. 7

Venn diagrammatic representation of the Pfam family distribution in shared and unique group of substrates. Venn diagrams were generated with http://bioinfogp.cnb.csic.es/tools/venny/.

Fig. 8

REMP motifs E. coli TorD aligned to identified archaeal TorD homologs.