Shigella surface protein IcsA and its cytoplasmic derivatives are localized to the old pole of rod-shaped cells when expressed in Escherichia coli. In spherical mreB cells, IcsA is targeted to ectopic sites and close to one extremity of actin-like MamK filament. To gain insight into the properties of the sites containing polar material, we studied the IcsA localization in spherical cells. GFP was exported into the periplasm via the Tat pathway and used as a periplasmic space marker. GFP displayed zonal fluorescence in both mreB and rodA-pbpA spherical K coli cells, indicating an uneven periplasmic space. Deconvolution images revealed that the cytoplasmic IcsA fused to mCherry was localized outside or at the edge of the GFP zones. These observations strongly suggest that polar material is restricted to the positions where the periplasm possesses particular structural or biochemical properties. (c) 2006 Elsevier Inc. All rights reserved.