Gene products of Amphiuma: an amphibian with an excessive amount of DNA

Structures of AAA protein translocase Bcs1 suggest translocation mechanism of a folded protein

The mitochondrial membrane-bound AAA protein Bcs1 translocate substrates across the mitochondrial inner membrane without previous unfolding. One substrate of Bcs1 is the iron-sulfur protein (ISP), a subunit of the respiratory Complex III. How Bcs1 translocates ISP across the membrane is unknown. Here we report structures of mouse Bcs1 in two different conformations, representing three nucleotide states. The apo and ADP-bound structures reveal a homo-heptamer and show a large putative substrate-binding cavity accessible to the matrix space. ATP binding drives a contraction of the cavity by concerted motion of the ATPase domains, which could push substrate across the membrane. Our findings shed light on the potential mechanism of translocating folded proteins across a membrane, offer insights into the assembly process of Complex III and allow mapping of human disease-associated mutations onto the Bcs1 structure.

Cell size and the concept of wasteful and frugal evolutionary strategies

It has been proposed by Cavalier-Smith that genome and cell size is controlled by the influence of r- and K-selection. This hypothesis, though explaining many regularities is unable to account for the enormous cells of some vertebrates, namely dipnoan fishes and caudate amphibians. The introduction of the concept of wasteful and frugal strategies unravels this problem. Wasteful strategy consists of evolution toward high metabolism, which is a prerequisite for high mobility and a large and complicated nervous system. Frugal strategy leads to economies of energy and material but constrains several evolutionary tendencies. It is much less common than the wasteful one and is especially prominent in fishes and amphibians living in tropical and temperate fresh waters where the respiratory conditions may force these animals to reduce their metabolism to a minimal level, or even to enter a period of aestivation. Since the increase in cell size diminishes the overall energy cost for the maintenance of ionic gradients between the cell interior and the intercellular solutions, it was favoured in dipnoans and in such caudate amphibians as inhabit fresh waters where an oxygen deficiency sometimes develops. The selection forces working for the diminution of genome and cell size may be very powerful and may change these cell characters in a comparatively short time, whereas small cells and genomes may be tolerated even in environments favouring a cell increase.