WPP Digital, the digital investment arm of advertising and public relations company WPP, has bought a minority stake in US group Mutual Mobile for an undisclosed sum.Mutual Mobile builds technology products for customers to improve communication and transactions across any mobile devices and all operating systems.It had revenue at December 31st 2012 of $25.7m (£16.5m). Clients include Google, Pearson, Cisco and Xerox. Based in Austin, the company employs 320 people. WPP said the deal would strengthen its digital media and mobile advertising business. WPP's digital revenues were well over $5bn in 2012 and represented 33% of total group revenues of $16.5bn. WPP Digital comprises the agencies Blue State Digital, Possible, Rockfish and F.biz in Brazil; the technology companies 24/7 Media, Acceleration, Hogarth and Salmon; as well as investments in technology companies such as eCommera, mySupermarket and Fullscreen.PW