apt-get install no public key

I’m trying to install apache thru apt-get, but it shows:-
W: There is no public key available for the following key IDs:
4D270D06F42584E6
W: You may want to run apt-get update to correct these problems

Even i run apt-get update, it wont solve the problem. after going thru some forum and mailing list, i finally found the solution.
To fix [...]