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Polymorphic membrane protein (Pmp)21 otherwise known as PmpD is the longest of 21 Pmps expressed by Chlamydophila pneumoniae. Recent bioinformatical analyses annotated PmpD as belonging to a family of exported Gram‐negative bacterial proteins designated autotransporters. This prediction, however, was never experimentally supported, nor was the function of PmpD known. Here, using 1D and 2D PAGE we demonstrate that PmpD is processed into two parts, N‐terminal (N‐pmpD), middle (M‐pmpD) and presumably third, C‐terminal part (C‐pmpD). Based on localization  of  the  external  part  on  the  outer  membrane as shown by immunofluorescence, immuno‐electron microscopy and immunoblotting combined with trypsinization, we demonstrate that N‐pmpD translocates to the surface of bacteria where it non‐covalently binds other components of the outer membrane. We propose that N …