Fig. 5: Diagram to summarize the function of BID1 in nodule cell ER reconfiguration and protein secretion.
From: A signal peptide peptidase is required for ER-symbiosome proximal association and protein secretion
Nodule cells secrete a large number of proteins, e.g., NCR peptides, to symbiosomes to promote their differentiation. SP sequences could be found at the N terminus of these host proteins. The DNF1 nodule-specific SPC cleaves SP fragments from nascent sequences of host proteins to ensure proper folding and SYP132-mediated target protein secretion. On the ER membrane, remnant SP fragments will be cleaved by BID1, the nodule-specific SPP. In WT cells containing symbiosomes, ER forms an extensive web-like structure, closely surrounding differentiated symbiosomes. In bid1 mutant, with the nodule-specific SPP missed, ER structural reconfiguration and ER-symbiosomes proximal association are damaged, resulting in blockage of host protein secretion. In summary the nodule-specific SPP on ER membrane is critical for ER structural reconfiguration, ER-symbiosome proximal association, and host protein secretion.
