Innate immune adapter that mediates the recruitment and activation of IRF5 downstream of endolysosomal toll-like receptors TLR7, TLR8 and TLR9. Following recruitment to endolysosome by SLC15A4 downstream of TLR7, TLR8 and TLR9, specifically recruits IRF5 transcription factor via its pLxIS motif, leading to IRF5 activation and subsequent expression of type I interferons. Plays a role in the regulation of endolysosomal pH in immune cells such as B-cells, dendritic cells and monocytes.