Fig. 4: Syt13 localizes to the apical PM of epithelial and the front domain of egressing endocrine cells.

a Syt13 localizes to the apical PM in polarized MDCK cells. b Western blot showing the close proximity of Syt13 with different apical proteins in MDCK cells. c Surface biotinylation assay showing Syt13 localization to the plasma membrane. d Apical localization of Syt13 full-length and the C2 truncated variants. e Quantification of apical localization of Syt13 protein variants. n = 4 independent experiments for d and e. f Scheme of positioning of an epithelium-residing endocrine precursor and an egressing endocrine cell. Distal (front) and proximal (rear) domains to the central lumen are defined in egressing endocrine cells. Created with BioRender.com. g Syt13 localizes to the apical PM (arrowhead) in differentiated endocrine cells fully residing within the pancreatic epithelium. Syt13 localizes at the front domain (arrowheads) of endocrine cells that are not in direct contact with the epithelial lumen in 3D epithelial cysts (h) and in vivo (i). j Pancreatic section showing the expression of aPKC at the front domain (arrowhead) of egressing endocrine cells. k Quantification of Syt13 and aPCK expression at the distal and proximal domains of newly formed endocrine cells. For Syt13, n = 54 endocrine cells from five independent pancreata (i and k). For aPKC, n = 38 endocrine cells from four independent pancreata (j and k). (l) Quantification of aPCK expression at the distal and proximal domains of newly formed endocrine cells in control and Syt13 KO pancreatic sections. For control, n = 43 endocrine cells from 4 independent pancreata. For KO n = 36 endocrine cells from three independent pancreata. m Binding of purified Syt13-C2AB protein variant to 100 nm sized LUVs containing different species of phosphatidylinositol phospholipids (POPC/cholesterol/phosphoinositide 65/30/5 mol %) assessed by electrochemiluminescence-based immunoassay (EIA). n = 3 independent experiments. n, o Treatment of explant culture of E13.5 Ngn3^Cre/+; ROSA26^mTmG/mTmG pancreata with PIP3 inhibitor PIT-1 for 48 h impairs endocrine cell egression. n = 3 independent experiments. Arrowheads indicate endocrine cells. L lumen. Scale bar 10 µm (d, g–j); 20 µm (a, n). Representative pictures from 2 (b, c) and 3 (a, g, h) independent experiments. Tukey’s multiple comparisons test (e) and two-sided t-test (k, l, o) were used for statistics. Data are represented as mean ± SD. Source data are provided as a Source Data file.