Dynamic Distribution of the SecA and SecY Translocase Subunits and Septal Localization of the HtrA Surface Chaperone/Protease during Streptococcus pneumoniae D39 Cell Division
Authors: Tsui Ho-Ching Tiffany , Keen Susan K. , Sham Lok-To , Wayne Kyle J. , Winkler Malcolm E. 
Published: October 2011

The Sec translocase pathway is the major route for protein transport across and into the cytoplasmic membrane of bacteria. Previous studies reported that the SecA translocase ATP-binding subunit and the cell surface HtrA protease/chaperone formed a single microdomain, termed “ExPortal,” in some species of ellipsoidal (ovococcus) Gram-positive bacteria, including
Streptococcus pyogenes
. To investigate the generality of microdomain formation, we determined the distribution of SecA and SecY by immunofluorescent microscopy in
Streptococcus pneumoniae
(pneumococcus), which is an ovococcus species evolutionarily distant from
S. pyogenes
. In the majority (≥75%) of exponentially growing cells,
S. pneumoniae
SecA (SecA

Spn

) and SecY

Spn

located dynamically in cells at different stages of division. In early divisional cells, both Sec subunits concentrated at equators, which are future sites of constriction. Further along in division, SecA

Spn

and SecY

Spn

remained localized at mid-cell septa. In late divisional cells, both Sec subunits were hemispherically distributed in the regions between septa and the future equators of dividing cells. In contrast, the HtrA

Spn

homologue localized to the equators and septa of most (>90%) dividing cells, whereas the SrtA

Spn

sortase located over the surface of cells in no discernable pattern. This dynamic pattern of Sec distribution was not perturbed by the absence of flotillin family proteins, but was largely absent in most cells in early stationary phase and in ∆
cls
mutants lacking cardiolipin synthase. These results do not support the existence of an ExPortal microdomain in
S. pneumoniae
. Instead, the localization of the pneumococcal Sec translocase depends on the stage of cell division and anionic phospholipid content.

IMPORTANCE
Two patterns of Sec translocase distribution, an ExPortal microdomain in certain ovococcus-shaped species like
Streptococcus pyogenes
and a spiral pattern in rod-shaped species like
Bacillus subtilis
, have been reported for Gram-positive bacteria. This study provides evidence for a third pattern of Sec localization in the ovococcus human pathogen
Streptococcus pneumoniae
. The SecA motor and SecY channel subunits of the Sec translocase localize dynamically to different places in the mid-cell region during the division cycle of exponentially growing, but not stationary-phase,
S. pneumoniae
. Unexpectedly, the
S. pneumoniae
HtrA (HtrA

Spn

) protease/chaperone principally localizes to cell equators and division septa. The coincident localization of SecA

Spn

, SecY

Spn

, and HtrA

Spn

to regions of peptidoglycan (PG) biosynthesis in unstressed, growing cells suggests that the pneumococcal Sec translocase directs assembly of the PG biosynthesis apparatus to regions where it is needed during division and that HtrA

Spn

may play a general role in quality control of proteins exported by the Sec translocase.