OBSOLETE. Catalysis of the formation of ice crystals in extracellular fluid at relatively high temperatures (up to -2 degrees C) to protect the organism from damage by intracellular ice formation. Ice nucleation proteins function by binding an ice crystal and then encouraging it to form larger crystals. Ice nucleation is a chemical process but these proteins can positively regulate it. There are two different uses of ice nucleation proteins: bacteria secrete them extracellularly to cause a host organism's cells to freeze and die, and fish use them to protect themselves from intracellular ice formation.