Desktop app store won't connect under proxy

My company recently instituted a corporate proxy and a few apps have been having difficulty with it. The app store is one of these. It just won't connect to anything - even the Help option just sits there "downloading latest help".

I have set up in my Network preference pane to connect HTTP and HTTPS to our proxy on port 80. Is there another port I need to set up here so the app store can connect as well?