Astrotactins 1 and 2 have similar membrane topology and mature via endoproteolytic cleavage catalyzed by signal peptidase
Astrotactins 1 and 2 (Astn1, Astn2) are membrane proteins that function in glial-guided migration, receptor trafficking and synaptic plasticity in brain, as well as in planar polarity pathways in skin. Here, we have mapped the topologies of mouse Astn1 and Astn2 in rough microsomal membranes (RMs) using glycosylation mapping and protease-protection assays, and find that Astn2 has a cleaved N-terminal signal peptide (SP), an N-terminal domain located in the lumen of the RMs (topologically equivalent to the extracellular surface in cells), two transmembrane helices (TMHs), and a large C-terminal lumenal domain. We find that Astn1 has the same topology as Astn2 but see no evidence of SP cleavage. Both Astn1 and Astn2 mature through endoproteolytic cleavage in the second TMH; importantly, we identify the endoprotease responsible for the maturation of Astn1 and Astn2 as signal peptidase. Differences in the degree of the maturation of Astn1 and Astn2 possibly contribute to the higher levels of the C-terminal domain of Astn1 detected on the CNS neuronal membranes and to the different functions of Astn1 and Astn2.