1.S.1.1.9
Carboxysome shell protein, CcmK4, of 113 aas.  It is a minor shell protein component of the carboxysome, a polyhedral inclusion where RuBisCO is sequestered. The central pore probably regulates metabolite flux, as might the gaps between assembled homohexamers (Kerfeld et al. 2005).  Homohexamers make sheets that probably form the facets of the polyhedral carboxysome (Dryden et al. 2009).  This subunit probably makes both homohexamers and heterohexamers with CcmK3 (Sommer et al. 2019).  Trettel et al. 2024 have summarized the properties of carboxysomes including the pore-forming shell protein subunits, ComK4, and considered how that can be useful for biotechnological purposes. They have also compared the shell proteins of carboxysomes with those of other metabolic compartments. Their models predict the biophysical properties surrounding the central pore in BMC-H shell subunits, which in turn dictate the efficiency of substrate diffusion (Trettel et al. 2024).