Plugging the transport gap

摘要

Molecular chaperones are generally thought to protect newly synthesized proteins and ensure that they fold into the correct shape. But it seems that two chaperones also help to target certain proteins to mitochondria. Mitochondria are the major energy-producing organelles of animal and fungal cells, and contain more than 700 different proteins. Fewer than 5% of these proteins are encoded by mitochondrial genes and produced inside mitochondria. Instead, most are encoded by genes in the cell nucleus. They are synthesized as preproteins in the cytoplasm by complex molecular machines called ribosomes, before being transported into developing mitochondria. Writing in Cell, Young et al. report that the transport of certain preproteins into mitochondria requires the chaperone proteins Hsp70 and Hsp90. The chaperones act by binding preproteins to a specific structure in a receptor protein called Tom70, which lies in the outer mitochondrial membrane (Fig. 1, overleaf). This paper helps to plug the gap left by two reports from the early years of 'chaperonology', which indicated that Hsp70 is involved in transporting proteins into mitochondria, although the transport mechanism was unknown. It also provides the first evidence that Hsp90 is involved in targeting proteins to a specific organelle.