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Transporting epithelia of insects are unlike most vertebrate epithelia in that they lack a serosa and possess septate junctions. The Na+/K+ pump is absent and an electrogenic K+ uniport pump is present in such insect epithelia as lepidopteran midgut, dipteran salivary glands, and many Malpighian tubules. The K+ pump is located in the apical plasma membrane and pumps K+ out of the cells. In midgut the transepithelial K+ transport is against a potential difference (PD) in excess of 120 mV and against a 10-fold K+ concentration difference in vivo. The pump uses a K+-modulated ATPase thought to be located in particles called K+ portasomes, which resemble the F1-F0 ATPase of phosphorylating membranes. Like F1-F0 particles the K+ portasomes are located on the cation input, electronegative, ATP-binding side of the membrane and appear to pump two cations for each MgATP2- hydrolyzed. We propose that in K+-transporting epithelia and in phosphorylating membranes running backwards the portasomes orient the binding of ATP with respect to a cation-gated channel in such a way that when MgATP2- is hydrolyzed P-i is separated from MgADP-; the 2 K+ or 2 H+ ions are no longer neutralized and are repelled from the channel to the opposite side of the membrane. We have isolated the K+ portasome-containing goblet cell apical membrane from larval Manduca sexta midgut and are attempting to isolate the K+ portasomes and K+-ATPase.

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