Woo!!! GREAT observation.

yes, when integral proteins must be embedded in place in the phospholipid bilayer, the alpha helices anchor the protein to the membrane.

but in order to create "beta barrel" structures for the passing of polar molecules, beta pleated sheets are needed.

So for a channel protein, you will find both alpha helix and beta sheet structures.

But, here's the thing

secondary protein structures are motifs, meaning these patterns are extremely common

so much so that you'd be hard pressed to find a protein that does NOT have alpha or beta secondary structures.

Most proteins, assuming they're made up of several thousands if not millions of amino acids,

will have multiple alpha helix and beta sheet structures throughout the entire protein.

So the fact that both alpha helix and beta sheet structures are found in one channel protein

is not something that is unique to channel proteins per se;

it's a feature that is shared by most proteins above a certain size.

Anyway, awesome observation, it's clear you are on top of your game.

Keep up the good work and let me know if you have any other questions. :D