The authors determined the first high resolution structure of an ion channel, called KcsA from
the bacterium Streptomyces lividans. The structure that they revealed was perfectly adapted to allow entry of potassium ions while excluding smaller sodium ions, thus explaining the high K+ selectivity and the high transport rate for the first time.

In 2003 MacKinnon received half of the Nobel Prize in Chemistry for "for structural and mechanistic studies of ion channels."

Order of authorship in the original paper: Doyle, Cabral...MacKinnon.

(Thanks to Juan Weiss for this reference and its interpretation.)

The authors first obtained the crystal structure of a Ca2+ - gated K+ ion channel and then deciphered how ion channels open and close, a process called "gating" in response to cues in their environment. They determined the structure of the opened state and revealed the complex molecular nature of the way gating transitions occur.
In 2003 Roderick MacKinnon shared the Nobel Prize in Chemistry Peter Agre with  “for discoveries concerning channels in cell membranes."

Order of authorship in the original publication: Jiang, Lee, Chen...MacKinnon.

(Thanks for Juan Weiss for this reference and its interpretation.)