<p>Source data are provided in S1 Data.xlsx.</p>

They typically assist in maintaining client proteins in a folded state or form pore channels during secretion. However, how OM proteins facilitate the secretion of proteins that lack classical signal peptides remains elusive.

Here, we demonstrate that they contribute to the secretion of unconventional B-type dye-decolorizing peroxidase (DypB₂₉₈₅) in <i>Pseudomonas putida</i>. The lipoprotein, Lpp₁₅₂₈, which contains a Sec signal peptide, is translocated to the periplasm through the Sec pathway and anchored in the inner leaflet of the OM. Lpp₁₅₂₈ recognizes the C-terminal hydrophobic region of DypB₂₉₈₅ in the cytoplasm and facilitates its coupling to the Sec machinery for inner membrane translocation, despite DypB₂₉₈₅ lacking a canonical N-terminal Sec signal peptide.