In this issue of Structure, Pulido et al. (2018) determine the crystal structure of procollagen C-proteinase enhancer-1 (PCPE-1)/procollagen III complex and identify that PCPE-1 unwinds the stalk of the procollagen III trimer, liberating a single chain to facilitate binding and cleavage by BMP-1 proteinases for subsequent fibrillar collagen assembly.
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