Transmembrane topography of the H -ATPase

The results of all of these topography studies are summarized in Fig. 1. The open eircles indicate residues of the H -ATPase shown in one way or another to be located on the cytoplasmic side of the membrane and the closed circles indicate residues in membrane-embedded segments. The lines in the sequence indicate minor regions with locations as yet not established. Thus, the topographical locations of nearly all of the 919 residues in the molecule have been established. It should be emphasized that the exact points of entry and exit of the polypeptide chain into and out of the membrane are not implied in the model. 

A first-generation model for the tertiary structure of the -ATPase 

With this information in hand, we may now consider how the -ATPase polypeptide chain might fold into its functional three-dimensional structure. First, regarding the actual number of membrane-spanning stretches, the available experimental data indicate only that each of the three membrane-embedded peptides must have an even number of and a minimum of two such stretches. However, hydropathy analysis by the method of Mohana Rao and Argos [48] suggests that the second mem- 

Except for the two additional membrane-spanning stretches in the second membrane-embedded segment, the proposed membrane-embedded stretches indicated in Fig. 1 are similar to those proposed for the closely related Ca -ATPase [53]. In fact, the overall topography proposed in Fig. 1 is quite similar to the Ca -ATPase model, lending additional credibility to each. 

Although this rendering is highly speculative, several features are worth noting briefly. 

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Fig. 1. Model for the transmembrane topography of the H -ATPase. OUT and IN indicate points of reference outside and inside an intact cell, respectively. See text for additional details.

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