RNA-binding protein that binds to a cis-localization element or zipcode, within the 5'-CDS of prolamine RNA (PubMed:20217123). Binds strongly to glutelin and prolamin mRNAs, particularly to 3'-UTR and zipcode RNA (PubMed:30659066). Recognizes and binds to glutelin zipcode RNA, which is required for proper mRNA localization to cisternal endoplasmic reticulum (PubMed:30659066). Recognizes and binds to prolamin zipcode RNA, which is required for proper mRNA localization to the protein body endoplasmic reticulum that delimits the prolamine intracisternal inclusion granules (PubMed:30659066). Required for the correct localization of glutelin and prolamine mRNA in endosperm cells during grain development (PubMed:30659066). RBP-L and RBP-P form a quaternary complex with the membrane trafficking factors NSF and RAB5A (PubMed:32471860). This quaternay complex carries glutelin mRNAs for active transport on endosomes to the cortical endoplasmic reticulum membrane, and enables endosome-mediated glutelin mRNA transport in endosperm cells (PubMed:32471860). Interacts with RBP-P (PubMed:30190374). Interacts with RAB5A (PubMed:32471860). Belongs to the polyadenylate-binding RBP45 family. Truncated N-terminus.