Astrotactin-1 and 2 (ASTN1/2) are integral membrane proteins with a large C-terminal domain, extracellular for ASTN1 and endosome luminal for ASTN-2 [1,2]. They play critical roles in neurodevelopment, and ASTN-2 is also involved in the planar cell polarity pathway in hair cells. This is a domain found in the middle of the C-terminal of ASTN-1 and 2, which comprises a fibronectin type III (Fn(III)) domain [1]. This domain located between the MACPF and annexin domains. The structure of Fn(III) from ASTN2 revealed an unexpected feature which has two additional beta strands folded across the core. The junction between EGF-4 and Fn(III) domains in ASTN2 (but not in ASTN1) is thought to be an inositol triphosphate binding site [1].

1: Structure of astrotactin-2: a conserved vertebrate-specific and perforin-like membrane protein involved in neuronal development. Ni T, Harlos K, Gilbert R; Open Biol. 2016; [Epub ahead of print] PMID:27249642

2: Astn2, a novel member of the astrotactin gene family, regulates the trafficking of ASTN1 during glial-guided neuronal migration. Wilson PM, Fryer RH, Fang Y, Hatten ME; J Neurosci. 2010;30:8529-8540. PMID:20573900