Phospholipid transfer function of PTPIP51 at mitochondria‐associated ER membranes

In eukaryotic cells, mitochondria are closely tethered to the endoplasmic reticulum (ER) at sites called mitochondria‐associated ER membranes (MAMs). Ca²⁺ ion and phospholipid transfer occurs at MAMs to support diverse cellular functions. Unlike those in yeast, the protein complexes involved in phospholipid transfer at MAMs in humans have not been identified. Here, we determine the crystal structure of the tetratricopeptide repeat domain of PTPIP51 (PTPIP51_TPR), a mitochondrial protein that interacts with the ER‐anchored VAPB protein at MAMs. The structure of PTPIP51_TPR shows an archetypal TPR fold, and an electron density map corresponding to an unidentified lipid‐like molecule probably derived from the protein expression host is found in the structure. We reveal functions of PTPIP51 in phospholipid binding/transfer, particularly of phosphatidic acid, in vitro. Depletion of PTPIP51 in cells reduces the mitochondrial cardiolipin level. Additionally, we confirm that the PTPIP51–VAPB interaction is mediated by the FFAT‐like motif of PTPIP51 and the MSP domain of VAPB. Our findings suggest that PTPIP51 is a phospholipid transfer protein with a MAM‐tethering function.     

Antisense inhibition of a spermidine synthase gene highlights the role of polyamines for stress alleviation in pear shoots subjected to salinity and cadmium

Three transgenic European pear (Pyrus communis L.) lines with reduced spermidine synthase (SPDS) expression and spermidine (Spd) titers were developed using a construct containing an apple SPDS gene (MdSPDS1) in antisense orientation. After exposure to either salt or cadmium stress, growth inhibition was more severe in the antisense lines than in the wild-type (WT). The antioxidant system, as shown by glutathione (GSH) content, activity of glutathione reductase (GR) and superoxide dismutase (SOD), and proline accumulation, was not effectively induced under stress in the antisense lines as compared with the WT. The reduction in antioxidant system function in the antisense lines was accompanied by a greater accumulation of malondialdehyde (MDA), an indicator of lipid peroxidation. Growth inhibition, Spd level, and parameters indicative of the antioxidant system were significantly ameliorated by exogenous Spd application. Under either salt or cadmium stress, GSH content, GR and SOD activity, and proline accumulation were positively correlated with Spd, putrescine (Put), and total polyamine titers. Conversely, MDA level showed a significantly negative correlation with these polyamines under both stress conditions. Thus, the responses to stress treatments were first identified in the SPDS antisense European pears, and the results provide further evidence for the important role of polyamines in both salt and cadmium stress tolerance, in which the polyamines act, at least in part, by influencing the antioxidant system.