Due to the changes in the underlying infrastructure between releases, you cannot save bus objects (or any other data type object) in a MAT-file in R2013b and reload them in an older release.
You will have to save your bus objects to a MATLAB script or function and then execute that in an older release to recreate the bus objects.
These are some of methods you can use to do that:
1) Open the Bus Editor from the command line by typing
at the MATLAB Command Prompt.
Then go to File->Export to File and save all the bus objects in your workspace as a MATLAB file( .m). It will then ask you if you want to save it as a cell or an object. Choose object.
You can then execute this MATLAB function generated to redefine these bus objects in any release that you want to.
2) Another alternative would be to use the SIMULINK.BUS.SAVE function. You can provide the filename and format (which is object) as parameters to this function.
The following is the documentation link for this function: