Can a professional corporation in Ontario be a shareholder of another corporation?

It depends on what that other corporation does. A professional corporation's business is generally restricted to practising its licensed profession, along with certain activities that are incidental to that practice — such as investing surplus funds from the practice. Whether holding shares in an unrelated operating business crosses the line depends on the specific restrictions your governing body places on the professional corporation's permitted activities, which vary by profession and aren't uniform across the board.

Some governing bodies take a fairly permissive view of passive investment holdings, such as a professional corporation owning shares in an investment holding company or a real estate entity, while being stricter about the corporation actively carrying on an unrelated business. Because the wrong structure can put the certificate of authorization at risk, don't assume a professional corporation can hold shares in another company the same way an ordinary business corporation can. Before setting up any such structure — for example, using the professional corporation to hold an investment portfolio or real estate — confirm it's permitted under your governing body's current rules and have a lawyer document it properly.