Proportionate mortality expresses the portion of all deaths due to a specific cause.
It is a proportion, not a rate.

Proportionate mortality = Deaths due to given cause / total deaths

Interpreting and using mortality data in humanitarian emergencies. A primer for non-epidemiologists.

F Checchi, L Roberts. Interpreting and using mortality data in humanitarian emergencies. Network Papers. Humanitarian Practice Network. No.52, Sep 2005. Internet. Accessed on October 9, 2012.