The V-ATPase in Paramecium:functional specialization by multiple gene isoforms

The vacuolar H(+)-ATPase (V-ATPase), a multisubunit, adenosine triphosphate (ATP)-driven proton pump, is essential for numerous cellular processes in all eukaryotes investigated so far. While structure and catalytic mechanism are similar to the evolutionarily related F-type ATPases, the V-ATPase's main function is to establish an electrochemical proton potential across membranes using ATP hydrolysis. The holoenzyme is formed by two subcomplexes, the transmembraneous V(0) and the cytoplasmic V(1) complexes. Sequencing of the whole genome of the ciliate Paramecium tetraurelia enabled the identification of virtually all the genes encoding V-ATPase subunits in this organism and the studying of the localization of the enzyme and roles in membrane trafficking and osmoregulation. Surprisingly, the number of V-ATPase genes in this free-living protozoan is strikingly higher than in any other species previously studied. Especially abundant are V(0)-a-subunits with as many as 17 encoding genes. This abundance creates the possibility of forming a large number of different V-ATPase holoenzymes by combination and has functional consequences by differential targeting to various organelles.

Optic flow in human vision: MEG reveals a foveo-fugal bias in V1, specialization for spiral space in hMSTs, and global motion sensitivity in the IPS

Abstract We recorded MEG responses from 17 participants viewing random-dot patterns simulating global optic flow components (expansion, contraction, rotation, deformation, and translation) and a random motion control condition. Theta-band (3–7 Hz), MEG signal power was greater for expansion than the other optic flow components in a region concentrated along the calcarine sulcus, indicating an ecologically valid, foveo-fugal bias for unidirectional motion sensors in V1. When the responses to the optic flow components were combined, a decrease in MEG beta-band (17–23 Hz) power was found in regions extending beyond the calcarine sulcus to the posterior parietal lobe (inferior to IPS), indicating the importance of structured motion in this region. However, only one cortical area, within or near the V5/hMT+ complex, responded to all three spiral-space components (expansion, contraction, and rotation) and showed no selectivity for global translation or deformation: we term this area hMSTs. This is the first demonstration of an exclusive region for spiral space in the human brain and suggests a functional role better suited to preliminary analysis of ego-motion than surface pose, which would involve deformation. We also observed that the rotation condition activated the cerebellum, suggesting its involvement in visually mediated control of postural adjustment.

Topography of the visual evoked magnetic response and hemispherical specialization

The visual evoked magnetic response CIIm component to a pattern onset stimulus presented half field produced a consistent scalp topography in 15 normal subjects. The major response was seen over the contralateral hemisphere, suggesting a dipole with current flowing away from the medial surface of the brain. Full field responses were more unpredictable. The reponses of five subjects were studied to the onset of a full, left half and right half checkerboard stimuli of 38 x 27 min arc checks appearing for 200 ms. In two subjects the full field CIIm topography was consistent with that of the mathematical summation of their relevant half field distribution. The remaining subjects had unpredictable full field topographies, showing little or no relationship to their half or summated half fields. In each of these subjects, a distribution matching that of the summated half field CIIm distribution appears at an earlier latency than that of the predominant full field waveform peak. By examining the topography of the full and half field responses at 5 ms intervals along the waveform for one such subject, the CIIm topography of the right hemisphere develops 10 ms before that of the left hemisphere, and is replaced by the following CIIIm component 20 ms earlier. Hence, the large peak seen in full field results from a combination of the CIIm component of the left hemisphere plus that of the CIIIm from the right. The earlier peak results from the CIIm generated in both hemispheres, at a latency where both show similar amplitudes. As the relative amplitudes of these two peaks alter with check and field size, topographic studies would be required for accurate CIIm identification. In addition. the CIIm-CIIIm complex lasts for 80 ms in the right hemisphere and 135 ms in the left, suggesting hemispherical apecialization in the visual processing of the pattern onset response.

Seventeen a-subunit isoforms of paramecium V-ATPase provide high specialization in localization and function

In the Paramecium tetraurelia genome, 17 genes encoding the 100-kDa-subunit (a-subunit) of the vacuolar-proton-ATPase were identified, representing by far the largest number of a-subunit genes encountered in any organism investigated so far. They group into nine clusters, eight pairs with >82% amino acid identity and one single gene. Green fluorescent protein-tagging of representatives of the nine clusters revealed highly specific targeting to at least seven different compartments, among them dense core secretory vesicles (trichocysts), the contractile vacuole complex, and phagosomes. RNA interference for two pairs confirmed their functional specialization in their target compartments: silencing of the trichocyst-specific form affected this secretory pathway, whereas silencing of the contractile vacuole complex-specific form altered organelle structure and functioning. The construction of chimeras between selected a-subunits surprisingly revealed the targeting signal to be located in the C terminus of the protein, in contrast with the N-terminal targeting signal of the a-subunit in yeast. Interestingly, some chimeras provoked deleterious effects, locally in their target compartment, or remotely, in the compartment whose specific a-subunit N terminus was used in the chimera.