how to cleave His-tag

[V. Yee]

We'd like to engineer a cleavage site to remove the his-tag from
our protein but cannot use a thrombin, fXa, or enterokinase site 
since these are calcium-requiring enzymes and we need to keep calcium 
from our protein.  Does anyone have a suggestion for a non-calcium
dependent protease that we can use, and which has been successfully
used by others (we are considering trypsin and chymotrypsin, but
are likely to have cleavage at more than the one desired site)?

Many thanks!
Vivien yee