What is the role of these gradient in SPA tag purification? - (Mar/27/2006 )

Dear all,
I am goin gto do a tag purification to get know protein protein interaction.
basically it is a two step purification. first step is a IP using a antibody-agarose, the solution is
10mM Tris Hcl, ph 8
100mM Nacl
0.1% triton-100
and the second step purification using
10mM Tris Hcl, ph 8
100mM Nacl
1mM imidazole
1mM Mg-Actate
2mM Cacl2
10mM p-mec
0.1% triton-100
and the elution is simple, I do not mention it here.
looks lot staff in the second step, I just what to know what is the role of the components in the buffer. thanks a lot.

cathy

by the way,
some guy suggested that all experiment carry out in RIPA lysis buffer. what do you think?
I have no clue which is better than which