Membrane binding mechanism of yeast mitochondrial peripheral membrane protein TIM44.

The protein translocations across mitochondrial membranes are carried out by specialized complexes, the Translocase of Outer Membrane (TOM) and Translocase of Inner Membrane (TIM). TIM23 translocon is responsible for translocating the mitochondrial matrix proteins across the mitochondrial inner membrane. Tim44 is an essential, peripheral membrane protein in TIM23 complex. Tim44 is tightly associated with the inner mitochondrial membrane on the matrix side. The Tim44 C-Terminal Domain (CTD) functions as an Inner Mitochondrial Membrane (IMM) anchor that recruits the Presequence protein Associated Motor (PAM) to the TIM23 channel. Using X-ray crystallographic and biochemical data, we show that the N-terminal helices A1 and A2 of Tim44 - CTD are crucial for its membrane tethering function. Based on our data, we propose a model showing how the N-terminal A1 and A2 amphipathic helices can either expose their hydrophobic face during membrane binding or conceal it in the soluble form. Therefore, the A1 and A2 helices of Tim44 may function as a membrane sensor.