Accurate initiator tRNA and mRNA start codon selection during translation initiation is essential for ensuring the integrity of gene expression; mis‐initiation using an elongator tRNA or a sense codon can lead to proteins containing aberrant N‐termini or to reading‐frame errors that yield truncated and/or mis‐folded proteins. In bacteria, three essential initiation factors (IFs) bind to the small, 30S, ribosomal subunit and control the accuracy of translation initiation by regulating tRNA and codon selection into the 30S initiation complex (30S IC) and joining of the large, 50S, ribosomal subunit to the 30S IC to form an elongation‐competent 70S initiation complex. Although IFs ensure that 50S subunit joining is dependent on the presence of a correctly selected initiator tRNA and start codon within the 30S IC, the mechanism through which the IFs accomplish this remains elusive. Using fluorescently labeled IFs, tRNAs, and ribosomal subunits, we have recently developed several single‐molecule fluorescence resonance energy transfer signals that report on the conformational dynamics of the 30S IC and on the dynamics of 50S subunit joining to the 30S IC. Here we will present the results of our latest studies, which, when integrated with the available biochemical data, strongly suggest that tRNA‐ and codon‐dependent conformational dynamics of the IFs play a critical role in regulating the 50S subunit joining step of translation initiation.
Synaptotagmin (Syt) proteins comprise a 17-member family, many of which trigger exocytosis in response to calcium. Historically, most studies have focused on the isoform Syt-1, which serves as the primary calcium sensor in synchronous neurotransmitter release. Recently, Syt-7 has become a topic of broad interest because of its extreme calcium sensitivity and diversity of roles in a wide range of cell types. Here, we review the known and emerging roles of Syt-7 in various contexts and stress the importance of its actions. Unique functions of Syt-7 are discussed in light of recent imaging, electrophysiological, and computational studies. Particular emphasis is placed on Syt-7–dependent regulation of synaptic transmission and neuroendocrine cell secretion. Finally, based on biochemical and structural data, we propose a mechanism to link Syt-7’s role in membrane fusion with its role in subsequent fusion pore expansion via strong calcium-dependent phospholipid binding.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)