The methionine-rich domain of the 54 kDa subunit of signal recognition particle is sufficient for the interaction with signal sequences

The signal recognition particle (SRP) binds to signal sequences when they emerge from a translating ribosome and targets the complex of ribosome, nascent chain and SRP to the membrane of the rough endoplasmic reticulum (rER) allowing the co-translational translocation of the nascent chain. By photo-crosslinking it has been shown that the signal sequence of preprolactin(PPL) only interacts with the methionine-rich (NI) domain of the 54 kDa protein subunit (SRP54) of SRP. Here we show that (i) a signal-anchor sequence is likewise crosslinked only to the methionine-rich domain of SRP54,(ii) free SRP54 can interact with signal sequences independently of the other components of SRP, (iii) its M domain suffices to perform this function, and (iv) an essentially intact M domain is required for signal sequence recognition. Alkylation of the N+G domain in intact SRP54 with N-ethyl maleimide (NE), but not after cleavage with V8 protease, prevents the binding of a signal sequence to the M domain. This suggests a proximity between the N+G and M domains of SRP54 and raises the possibility that the role of the N+G domain may be to regulate the binding and/or the release of signal sequences

The intracellular accumulation of polymeric neuroserpin explains the severity of the dementia FENIB

Familial encephalopathy with neuroserpin inclusion bodies (FENIB) is an autosomal dominant dementia that is characterized by the retention of polymers of neuroserpin as inclusions within the endoplasmic reticulum (ER) of neurons. We have developed monoclonal antibodies that detect polymerized neuroserpin and have used COS-7 cells, stably transfected PC12 cell lines and transgenic Drosophila melanogaster to characterize the cellular handling of all four mutant forms of neuroserpin that cause FENIB. We show a direct correlation between the severity of the disease-causing mutation and the accumulation of neuroserpin polymers in cell and fly models of the disease. Moreover, mutant neuroserpin causes locomotor deficits in the fly allowing us to demonstrate a direct link between polymer accumulation and neuronal toxicity