Private galleries are becoming ever more serious about going public, increasingly hiring highly credentialed curators to put together 'museum quality' exhibitions. The enormous new Paris home for François Pinault's private haul, opening this autumn, ups the ante considerably. Athens' George Economou Collection is more modest in scale but sharp of focus.

A local shipping magnate, Economou began collecting in the 1990s, quickly assembling one of the world's best collections of German Expressionism - the Hermitage even turned to him to plug some their gaps in a recent exhibition - before adding more contemporary works. His gallery now puts on two or three shows a year, always with the involvement of international curatorial talent. The latest is 'Magic Numbers', a solo exhibition by the young African American artist Rashid Johnson.

Born in Chicago, Johnson uses paintings, sculpture, photography, video and found objects - including books and records - to explore questions of identity and particularly the legacy of afro-centrism. Recent works have taken a more abstract turn, using materials such as burned wood, cast bronze, black soap and wax.

The centrepiece of 'Magic Numbers', curated by Johnson in collaboration with Katherine Brinson, associate curator at the Guggenheim Museum in New York, and Skarlet Smatana, director of the Economou Collection, is the film 'The New Black Yoga' (2011), which is almost exactly what you would expect: footage of a group of black men doing yoga on a beach. Around this Johnson, whose installations always suggest the domestic and a kind of origin story, has grouped a series of new paintings and sculptures.