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Fig. 4 | BMC Biology

Fig. 4

From: Competition between type I activin and BMP receptors for binding to ACVR2A regulates signaling to distinct Smad pathways

Fig. 4

ALK4 competes with BMP type I receptors for binding ACVR2A. Experimental conditions were as in Fig. 3, with patch/FRAP measurements conducted on cells expressing myc-ACVR2A alone or together with HA-ALK6 or HA-ALK2 (instead of HA-ALK4). Where indicated, untagged ALK4 was coexpressed in addition to the myc- and HA-tagged receptors as a competitor. a, c Average Rf values; b, d Average D values. Bars are mean ± SEM; the number of measurements is depicted on each bar. The full and dashed lines show for comparison the Rf values of myc-ACVR2A coexpressed only with HA-ALK6 (a) or HA-ALK2 (c), as indicated to the right of the panels, without (full line) or with (dashed line) IgG αHA crosslinking of the indicated HA-tagged receptor (see Fig. 2f, j). No significant differences (one-way ANOVA with Bonferroni post hoc test; P > 0.5) were found between the D values under any condition (b and d). The reduction in Rf of myc-ACVR2A upon coexpression with HA-ALK6 or HA-ALK2 (with or without IgG crosslinking) disappeared following coexpression with untagged ALK4, and no significant differences were found from the Rf value of singly expressed myc-ACVR2A (P > 0.08, one-way ANOVA and Bonferroni post hoc test). These results indicate that ALK4 competes with HA-ALK2 or HA-ALK6 for binding myc-ACVR2A. e–h Point-confocal measurements of the expression levels of coexpressed receptors. e Level of myc-ACVR2A coexpressed with HA-ALK6, without or with untagged ALK4. f Level of HA-ALK6 with myc-ACVR2A alone or with untagged ALK4. g Level of myc-ACVR2A coexpressed with HA-ALK2 alone or with untagged ALK4. h Level of HA-ALK2 coexpressed with myc-ACVR2A alone or with untagged ALK4. Measurements were as described under “Methods.” Bars represents mean ± SEM of 30 independent measurements. No significant differences were observed between the levels of any receptor pairs compared (Student’s two-tailed t test; P > 0.5)

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