A case of imperfect pseudo-merohedral twinning in monoclinic crystals of fungal fatty acid synthase is discussed. A space-group transition during crystal dehydration resulted in a Moiré pattern-like interference of the twinned diffraction patterns.

The recent high-resolution structures of fungal fatty acid synthase (FAS) have provided new insights into the principles of fatty acid biosynthesis by large multifunctional enzymes. The crystallographic phase problem for the 2.6 MDa fungal FAS was initially solved to 5 Å resolution using two crystal forms from Thermomyces lanuginosus. Monoclinic crystals in space group P21 were obtained from orthorhombic crystals in space group P212121 by dehydration. Here, it is shown how this space-group transition induced imperfect pseudo-merohedral twinning in the monoclinic crystal, giving rise to a Moiré pattern-like interference of the two twin-related reciprocal lattices. The strategy for processing the twinned diffraction images and obtaining a quantitative analysis is presented. The twinning is also related to the packing of the molecules in the two crystal forms, which was derived from self-rotation function analysis and molecular-replacement solutions using a low-resolution electron microscopy map as a search model.

Secreted and membrane proteins are translocated across/into cell membranes via a protein-conducting channel (PCC). We present a cryo-EM reconstruction of the E. coli PCC, SecYEG, complexed with the ribosome and a signal anchor containing nascent chain, showing mRNA, three tRNAs, the nascent chain, and detailed features of both a translocating PCC and a second, non-translocating PCC bound to mRNA hairpins. The translocating PCC forms connections with ribosomal RNA hairpins on two sides and ribosomal proteins at the back, leaving a frontal opening. Normal mode-based flexible fitting of the archaeal SecYEβ structure into the PCC EM densities favors a front-to-front arrangement of two SecYEG complexes in the PCC, and supports channel formation by the opening of two linked SecY halves during polypeptide translocation. Based on our observation in the translocating PCC of two segregated pores with different degrees of access to bulk lipid, we propose a model for co-translational protein translocation.